tests, extract helpers to modify global state.

Make sure that tests do not hardcode the default value.
For example `test_instantiation_doesnt_try_to_require_corresponding_file`
always restored the configuration to `true` regardless of what it's
original value was.

Extract a helper to make the global modification consistent across tests.

activerecord/test/cases/inheritance_test.rb

@@ -7,15 +7,32 @@ require 'models/subscriber'
 require 'models/vegetables'
 require 'models/shop'
 
+module InheritanceTestHelper
+  def with_store_full_sti_class(&block)
+    assign_store_full_sti_class true, &block
+  end
+
+  def without_store_full_sti_class(&block)
+    assign_store_full_sti_class false, &block
+  end
+
+  def assign_store_full_sti_class(flag)
+    old_store_full_sti_class = ActiveRecord::Base.store_full_sti_class
+    ActiveRecord::Base.store_full_sti_class = flag
+    yield
+  ensure
+    ActiveRecord::Base.store_full_sti_class = old_store_full_sti_class
+  end
+end
+
 class InheritanceTest < ActiveRecord::TestCase
+  include InheritanceTestHelper
   fixtures :companies, :projects, :subscribers, :accounts, :vegetables
 
   def test_class_with_store_full_sti_class_returns_full_name
-    old = ActiveRecord::Base.store_full_sti_class
-    ActiveRecord::Base.store_full_sti_class = true
+    with_store_full_sti_class do
       assert_equal 'Namespaced::Company', Namespaced::Company.sti_name
-  ensure
-    ActiveRecord::Base.store_full_sti_class = old
+    end
   end
 
   def test_class_with_blank_sti_name
@@ -33,39 +50,31 @@ class InheritanceTest < ActiveRecord::TestCase
   end
 
   def test_class_without_store_full_sti_class_returns_demodulized_name
-    old = ActiveRecord::Base.store_full_sti_class
-    ActiveRecord::Base.store_full_sti_class = false
+    without_store_full_sti_class do
       assert_equal 'Company', Namespaced::Company.sti_name
-  ensure
-    ActiveRecord::Base.store_full_sti_class = old
+    end
   end
 
   def test_should_store_demodulized_class_name_with_store_full_sti_class_option_disabled
-    old = ActiveRecord::Base.store_full_sti_class
-    ActiveRecord::Base.store_full_sti_class = false
+    without_store_full_sti_class do
       item = Namespaced::Company.new
       assert_equal 'Company', item[:type]
-  ensure
-    ActiveRecord::Base.store_full_sti_class = old
+    end
   end
 
   def test_should_store_full_class_name_with_store_full_sti_class_option_enabled
-    old = ActiveRecord::Base.store_full_sti_class
-    ActiveRecord::Base.store_full_sti_class = true
+    with_store_full_sti_class do
       item = Namespaced::Company.new
       assert_equal 'Namespaced::Company', item[:type]
-  ensure
-    ActiveRecord::Base.store_full_sti_class = old
+    end
   end
 
   def test_different_namespace_subclass_should_load_correctly_with_store_full_sti_class_option
-    old = ActiveRecord::Base.store_full_sti_class
-    ActiveRecord::Base.store_full_sti_class = true
-    item = Namespaced::Company.create :name => "Wolverine 2"
+    with_store_full_sti_class do
+      item = Namespaced::Company.create name: "Wolverine 2"
       assert_not_nil Company.find(item.id)
       assert_not_nil Namespaced::Company.find(item.id)
-  ensure
-    ActiveRecord::Base.store_full_sti_class = old
+    end
   end
 
   def test_company_descends_from_active_record
@@ -331,6 +340,7 @@ class InheritanceTest < ActiveRecord::TestCase
 end
 
 class InheritanceComputeTypeTest < ActiveRecord::TestCase
+  include InheritanceTestHelper
   fixtures :companies
 
   def setup
@@ -344,7 +354,7 @@ class InheritanceComputeTypeTest < ActiveRecord::TestCase
   end
 
   def test_instantiation_doesnt_try_to_require_corresponding_file
-    ActiveRecord::Base.store_full_sti_class = false
+    without_store_full_sti_class do
       foo = Firm.first.clone
       foo.type = 'FirmOnTheFly'
       foo.save!
@@ -363,8 +373,7 @@ class InheritanceComputeTypeTest < ActiveRecord::TestCase
       # And instantiate will find the existing constant rather than trying
       # to require firm_on_the_fly.
       assert_nothing_raised { assert_kind_of Firm::FirmOnTheFly, Firm.find(foo.id) }
-  ensure
-    ActiveRecord::Base.store_full_sti_class = true
+    end
   end
 
   def test_sti_type_from_attributes_disabled_in_non_sti_class