Make test file patterns configurable via Environment variables

This makes test file patterns configurable via two environment variables:
`DEFAULT_TEST`, to configure files to test, and `DEFAULT_TEST_EXCLUDE`,
to configure files to exclude from testing.

These values were hardcoded before, which made it difficult to add
new categories of tests that should not be executed by default (e.g:
smoke tests).

It uses environment variables instead of regular Rails config options
because Rails environment is not available when the Runner builds the
list of files to test (unless using Spring). A nicer solution would be
making sure that the Rails environment is always loaded when the runner
starts. This is a first simple step to make these paths configurable for
now

This way at least you could override defaults in `config/boot.rb`:

```ruby
ENV["DEFAULT_TEST_EXCLUDE"] = "test/{dummy,smoke,system}/**/*_test.rb
```

Co-authored-by: Jeremy Daer <jeremydaer@gmail.com>

railties/CHANGELOG.md

@@ -1,3 +1,15 @@
+*   Make test file patterns configurable via Environment variables
+
+    This makes test file patterns configurable via two environment variables:
+     `DEFAULT_TEST`, to configure files to test, and `DEFAULT_TEST_EXCLUDE`,
+    to configure files to exclude from testing.
+
+    These values were hardcoded before, which made it difficult to add
+    new categories of tests that should not be executed by default (e.g:
+    smoke tests).
+
+    *Jorge Manrubia*
+
 *   No longer include `rake rdoc` task when generating plugins.
 
     To generate docs, use the `rdoc lib` command instead.

railties/lib/rails/test_unit/runner.rb

@@ -44,8 +44,9 @@ module Rails
         def load_tests(argv)
           patterns = extract_filters(argv)
 
-          tests = Rake::FileList[patterns.any? ? patterns : "test/**/*_test.rb"]
-          tests.exclude("test/system/**/*", "test/dummy/**/*") if patterns.empty?
+
+          tests = Rake::FileList[patterns.any? ? patterns : default_test_glob]
+          tests.exclude(default_test_exclude_glob) if patterns.empty?
 
           tests.to_a.each { |path| require File.expand_path(path) }
         end
@@ -76,6 +77,14 @@ module Rails
             end
           end
 
+          def default_test_glob
+            ENV["DEFAULT_TEST"] || "test/**/*_test.rb"
+          end
+
+          def default_test_exclude_glob
+            ENV["DEFAULT_TEST_EXCLUDE"] || "test/{system,dummy}/**/*_test.rb"
+          end
+
           def regexp_filter?(arg)
             arg.start_with?("/") && arg.end_with?("/")
           end

railties/test/application/test_runner_test.rb

@@ -895,6 +895,21 @@ module ApplicationTests
       assert_match "1 runs, 1 assertions, 0 failures, 0 errors, 0 skips", output
     end
 
+    def test_can_exclude_files_from_being_tested_via_default_rails_command_by_setting_DEFAULT_TEST_EXCLUDE_env_var
+      create_test_file "smoke", "smoke_foo"
+
+      switch_env "DEFAULT_TEST_EXCLUDE", "test/smoke/**/*_test.rb" do
+        assert_match "0 runs, 0 assertions, 0 failures, 0 errors, 0 skips", run_test_command("")
+      end
+    end
+
+    def test_can_exclude_files_from_being_tested_via_rake_task_by_setting_DEFAULT_TEST_EXCLUDE_env_var
+      create_test_file "smoke", "smoke_foo"
+
+      output = Dir.chdir(app_path) { `DEFAULT_TEST_EXCLUDE="test/smoke/**/*_test.rb" bin/rake test` }
+      assert_match "0 runs, 0 assertions, 0 failures, 0 errors, 0 skips", output
+    end
+
     private
       def run_test_command(arguments = "test/unit/test_test.rb", **opts)
         rails "t", *Shellwords.split(arguments), allow_failure: true, **opts