Refactor test folder and create dummy app

[invalidusrname]

The refactoring follows the rails guide approach for creating a rails plugin by using a dummy application to test the gem.

This allows all tests to be run under a single command:

$ bundle exec rspec

I reorganized tests so they all live under the spec folder.

Inspiration for this PR:

• https://corlewsolutions.com/articles/article-126-how-to-setup-rspec-in-a-rails-engine
• https://www.codementor.io/@mohnishjadwani/how-to-setup-rspec-factory-bot-and-spring-for-a-rails-5-engine-qjdpthfb1

NOTE: Please merge this in after #286