Isolate railties Generators Tests

With this change jruby should pass all of the railties tests.
Merge pull request #11726 from atambo/isolate_generator_tests

Isolate railties Generators Tests
Revert "Isolate railties Generators Tests"

This fixes railties tests in JRuby but break it in MRI
This reverts commit fea99276c7024f183bf72a16ad9f36af4bae1d42.