Use 'yarn link' in the test app

[taylor-steve]

Now that app.config.assets.paths << Rails.root.join('node_modules') has been removed via #3378 we can go back to using yarn link for the test app. It seems like that fixes our previous infinite propshaft issue.

This fixes the test app build, which is currently broken:

07:58:13 css.1  | Error: Can't find stylesheet to import.
07:58:13 css.1  |   ╷
07:58:13 css.1  | 5 │ @import "spotlight-frontend/vendor/assets/stylesheets/bootstrap-tagsinput";
07:58:13 css.1  |   │         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
07:58:13 css.1  |   ╵
07:58:13 css.1  |   app/assets/stylesheets/spotlight.scss 5:9              @import
07:58:13 css.1  |   app/assets/stylesheets/application.bootstrap.scss 4:9  root stylesheet
07:58:13 css.1  | error Command failed with exit code 65.
07:58:13 css.1  | info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.
07:58:13 css.1  | error Command failed with exit code 65.
07:58:13 css.1  | info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.
07:58:13 css.1  | [nodemon] app crashed - waiting for file changes before starting...