test: extract test app

[bf4]

I created new rails engine with a dummy app and copied that into our
test folder in the same place, test/dummy. I suspect that using the
incantantations in the generated test/dummy and test/test_helper.rb will
help get us to Rails 7.1 test parity.

rails plugin new TestApp --mountable

All Submissions:

• I've checked to ensure there aren't other open Pull Requests for the same update/change.
• I've submitted a ticket for my issue if one did not already exist.
• My submission passes all tests. (Please run the full test suite locally to cut down on noise from travis failures.)
  • except for Rails 7.1, which is already failing
• I've used Github auto-closing keywords in the commit message or the description.
• I've added/updated tests for this change.

New Feature Submissions:

• I've submitted an issue that describes this feature, and received the go ahead from the maintainers.
• My submission includes new tests.
• My submission maintains compliance with JSON:API.

Bug fixes and Changes to Core Features:

• I've included an explanation of what the changes do and why I'd like you to include them.
• I've provided test(s) that fails without the change.

Test Plan:

Reviewer Checklist:

• Maintains compliance with JSON:API
• Adequate test coverage exists to prevent regressions

[bf4]

was trying to solve the problem which https://github.com/cerebris/jsonapi-resources/pull/1420 solves, so closing since this is interesting, but not needed