To fix#43920, f292daad70 added
`sprockets-rails` to the generated `Gemfile` for engine plugins because
their dummy apps use Sprockets. However, non-engine plugins exhibit the
same issue because their dummy apps also use Sprockets.
This commit forces `skip_asset_pipeline` to be true when a plugin is not
an engine, and fixes several tests that failed to detect these issues
because they were accidentally using the `rails/rails` `Gemfile` instead
of the generated plugin `Gemfile`.
* require, require_relative, load by double quotes
We're getting rid of all single quote usage, unless it serves a specific purpose, as per the general style guide.
Since the absolute path is not required to re-run the test,
modified so that unnecessary information is not displayed.
```ruby
# before
bin/rails test /path/to/blorgh/test/integration/navigation_test.rb:5
# after
bin/rails test test/integration/navigation_test.rb:5
```
