While looking for templates using the `.rb` extension instead of
`.rb.tt`, my search ended up finding this `test_helper.rb` file.

```
$ fd | rg "/templates/.*\.rb$"
./actioncable/lib/rails/generators/channel/templates/application_cable/channel.rb
./actioncable/lib/rails/generators/channel/templates/application_cable/connection.rb
./railties/lib/rails/generators/test_unit/plugin/templates/test_helper.rb
```

Instead of correcting the extension, I opted to remove it because the
`test_unit:plugin` generator appears to be unused.

Co-Authored-By: Jean Boussier <jean.boussier@gmail.com>