bring back `TEST` env for `rake test`.

Closes #23027.

This does not restore complete backwards compatibility. It simply passes
the contets of the `TEST` env to the new runner.

railties/CHANGELOG.md

+*   Bring back `TEST=` env for `rake test` task.
+
+    *Yves Senn*
+
 *   Specify log file names or all logs to clear `rake log:clear`
 
     Specify which logs to clear when using the `rake log:clear` task, e.g. `rake log:clear LOGS=test,staging`

railties/lib/rails/test_unit/testing.rake

@@ -7,7 +7,12 @@ task default: :test
 desc "Runs all tests in test folder"
 task :test do
   $: << "test"
-  Minitest.rake_run(["test"])
+  pattern = if ENV.key?('TEST')
+               ENV['TEST']
+             else
+               "test"
+             end
+  Minitest.rake_run([pattern])
 end
 
 namespace :test do

railties/test/application/test_runner_test.rb

@@ -450,6 +450,17 @@ def test_raise_error_when_specified_file_does_not_exist
       assert_match(%r{cannot load such file.+test/not_exists\.rb}, error)
     end
 
+    def test_pass_TEST_env_on_rake_test
+      create_test_file :models, 'account'
+      create_test_file :models, 'post', pass: false
+
+      output =  Dir.chdir(app_path) { `bin/rake test TEST=test/models/post_test.rb` }
+
+      assert_match "PostTest", output, "passing TEST= should run selected test"
+      assert_no_match "AccountTest", output, "passing TEST= should only run selected test"
+      assert_match '1 runs, 1 assertions', output
+    end
+
     private
       def run_test_command(arguments = 'test/unit/test_test.rb')
         Dir.chdir(app_path) { `bin/rails t #{arguments}` }