tests, run `test_test.rb` with `RAILS_ENV=test`.

The tests to verify our testing setup used `RAILS_ENV=development`
to execute the tests. Let's keep it as close to a real-world setup
as possible.

railties/test/application/test_test.rb

@@ -208,7 +208,7 @@ def assert_successful_test_run(name)
       end
 
       def run_test_file(name, options = {})
-        ruby '-Itest', "#{app_path}/test/#{name}", options
+        ruby '-Itest', "#{app_path}/test/#{name}", options.deep_merge(env: {"RAILS_ENV" => "test"})
       end
 
       def ruby(*args)