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
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• 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 #1420 solves, so closing since this is interesting, but not needed