Fix inflector test by using dup inflections when it needs to be changed

This is not something that is failing at the moment, but can do it
eventually. I had the issue with db62081 as the HEAD and with the
following change:

```
--- a/activesupport/test/inflector_test.rb
+++ b/activesupport/test/inflector_test.rb
@@ -101,6 +101,7 @@ class InflectorTest < ActiveSupport::TestCase
   def test_acronyms
     ActiveSupport::Inflector.inflections do |inflect|
       inflect.acronym("API")
+      inflect.acronym("HTM")
       inflect.acronym("HTML")
       inflect.acronym("HTTP")
       inflect.acronym("RESTful")
```

I was expecting only `test_acronyms` to fail, but with a specific `seed`
others were also failing: `ruby -w -I"lib:test" test/inflector_test.rb
--seed 4313`.

Now, `inflections` instance is duplicated on `setup` and restored on
`teardown`.

I decided to benchmark and check the impact of the patch and it seems
to me to be fine.

```
Calculating -------------------------------------
     without changes     1.000  i/100ms
      with setup dup     1.000  i/100ms
      with block dup     1.000  i/100ms
-------------------------------------------------
     without changes      0.817  (± 0.0%) i/s -      5.000  in
     6.119916s
      with setup dup      0.784  (± 0.0%) i/s -      4.000
      with block dup      0.797  (± 0.0%) i/s -      4.000
```

Where `with setup dup` duplicates on setup for each test and `with block`
duplicates for just for tests that actually modify `inflections`.

activesupport/test/inflector_test.rb

@@ -8,6 +8,20 @@ class InflectorTest < ActiveSupport::TestCase
   include InflectorTestCases
   include ConstantizeTestCases
 
+  def setup
+    # Dups the singleton before each test, restoring the original inflections later.
+    #
+    # This helper is implemented by setting @__instance__ because in some tests
+    # there are module functions that access ActiveSupport::Inflector.inflections,
+    # so we need to replace the singleton itself.
+    @original_inflections = ActiveSupport::Inflector::Inflections.instance_variable_get(:@__instance__)[:en]
+    ActiveSupport::Inflector::Inflections.instance_variable_set(:@__instance__, en: @original_inflections.dup)
+  end
+
+  def teardown
+    ActiveSupport::Inflector::Inflections.instance_variable_set(:@__instance__, en: @original_inflections)
+  end
+
   def test_pluralize_plurals
     assert_equal "plurals", ActiveSupport::Inflector.pluralize("plurals")
     assert_equal "Plurals", ActiveSupport::Inflector.pluralize("Plurals")
@@ -26,7 +40,6 @@ class InflectorTest < ActiveSupport::TestCase
   end
 
   def test_uncountable_word_is_not_greedy
-    with_dup do
     uncountable_word = "ors"
     countable_word = "sponsor"
 
@@ -40,7 +53,6 @@ class InflectorTest < ActiveSupport::TestCase
     assert_equal "sponsors", ActiveSupport::Inflector.pluralize(countable_word)
     assert_equal "sponsor", ActiveSupport::Inflector.singularize(ActiveSupport::Inflector.pluralize(countable_word))
   end
-  end
 
   SingularToPlural.each do |singular, plural|
     define_method "test_pluralize_singular_#{singular}" do
@@ -70,12 +82,10 @@ class InflectorTest < ActiveSupport::TestCase
 
 
   def test_overwrite_previous_inflectors
-    with_dup do
     assert_equal("series", ActiveSupport::Inflector.singularize("series"))
     ActiveSupport::Inflector.inflections.singular "series", "serie"
     assert_equal("serie", ActiveSupport::Inflector.singularize("series"))
   end
-  end
 
   MixtureToTitleCase.each_with_index do |(before, titleized), index|
     define_method "test_titleize_mixture_to_title_case_#{index}" do
@@ -367,11 +377,9 @@ class InflectorTest < ActiveSupport::TestCase
   %w{plurals singulars uncountables humans}.each do |inflection_type|
     class_eval <<-RUBY, __FILE__, __LINE__ + 1
       def test_clear_#{inflection_type}
-        with_dup do
         ActiveSupport::Inflector.inflections.clear :#{inflection_type}
         assert ActiveSupport::Inflector.inflections.#{inflection_type}.empty?, \"#{inflection_type} inflections should be empty after clear :#{inflection_type}\"
       end
-      end
     RUBY
   end
 
@@ -405,7 +413,6 @@ class InflectorTest < ActiveSupport::TestCase
   end
 
   def test_clear_all
-    with_dup do
     ActiveSupport::Inflector.inflections do |inflect|
       # ensure any data is present
       inflect.plural(/(quiz)$/i, '\1zes')
@@ -421,10 +428,8 @@ class InflectorTest < ActiveSupport::TestCase
       assert inflect.humans.empty?
     end
   end
-  end
 
   def test_clear_with_default
-    with_dup do
     ActiveSupport::Inflector.inflections do |inflect|
       # ensure any data is present
       inflect.plural(/(quiz)$/i, '\1zes')
@@ -440,11 +445,9 @@ class InflectorTest < ActiveSupport::TestCase
       assert inflect.humans.empty?
     end
   end
-  end
 
   Irregularities.each do |singular, plural|
     define_method("test_irregularity_between_#{singular}_and_#{plural}") do
-      with_dup do
       ActiveSupport::Inflector.inflections do |inflect|
         inflect.irregular(singular, plural)
         assert_equal singular, ActiveSupport::Inflector.singularize(plural)
@@ -452,29 +455,24 @@ class InflectorTest < ActiveSupport::TestCase
       end
     end
   end
-  end
 
   Irregularities.each do |singular, plural|
     define_method("test_pluralize_of_irregularity_#{plural}_should_be_the_same") do
-      with_dup do
       ActiveSupport::Inflector.inflections do |inflect|
         inflect.irregular(singular, plural)
         assert_equal plural, ActiveSupport::Inflector.pluralize(plural)
       end
     end
   end
-  end
 
   Irregularities.each do |singular, plural|
     define_method("test_singularize_of_irregularity_#{singular}_should_be_the_same") do
-      with_dup do
       ActiveSupport::Inflector.inflections do |inflect|
         inflect.irregular(singular, plural)
         assert_equal singular, ActiveSupport::Inflector.singularize(singular)
       end
     end
   end
-  end
 
   [ :all, [] ].each do |scope|
     ActiveSupport::Inflector.inflections do |inflect|
@@ -503,7 +501,6 @@ class InflectorTest < ActiveSupport::TestCase
 
   %w(plurals singulars uncountables humans acronyms).each do |scope|
     define_method("test_clear_inflections_with_#{scope}") do
-      with_dup do
       # clear the inflections
       ActiveSupport::Inflector.inflections do |inflect|
         inflect.clear(scope)
@@ -511,7 +508,6 @@ class InflectorTest < ActiveSupport::TestCase
       end
     end
   end
-  end
 
   def test_inflections_with_uncountable_words
     ActiveSupport::Inflector.inflections do |inflect|
@@ -520,18 +516,4 @@ class InflectorTest < ActiveSupport::TestCase
 
     assert_equal "HTTP", ActiveSupport::Inflector.pluralize("HTTP")
   end
-
-  # Dups the singleton and yields, restoring the original inflections later.
-  # Use this in tests what modify the state of the singleton.
-  #
-  # This helper is implemented by setting @__instance__ because in some tests
-  # there are module functions that access ActiveSupport::Inflector.inflections,
-  # so we need to replace the singleton itself.
-  def with_dup
-    original = ActiveSupport::Inflector::Inflections.instance_variable_get(:@__instance__)[:en]
-    ActiveSupport::Inflector::Inflections.instance_variable_set(:@__instance__, en: original.dup)
-    yield
-  ensure
-    ActiveSupport::Inflector::Inflections.instance_variable_set(:@__instance__, en: original)
-  end
 end