Use dynamic path for dummy app location in plugin's test_helper.rb

Currently if you specify a dummy-path options when you create a plugin, the test for the plugin doesn't work.
```bash
> rails plugin new my_plugin --dummy-path=my/dummy/app
> cd my_plugin
> bundle exec rake test
/home/yukio/experiments/rails_gem/tmp/my_plugin/test/test_helper.rb:4:in `require': cannot load such file --
 /home/yukio/experiments/rails_gem/tmp/my_plugin/test/dummy/config/environment.rb (LoadError)
```
This patch will fix the cases like this. Even if you specify your own dummy_path, the test will work correctly.