Make turbo_test.rb consistent with Rails' generated test_helper.rb

[seanpdoyle]

I've opened this as a draft PR, since I'm unsure about the root-cause of database records leaking across test runs

Something in the test suite configuration is preventing the database
from being wiped between test runs. This results in state leaking
between tests. As a result, our Continuous Integration tests are flaky.

-- hotwired/turbo-rails#248

As a follow-up to the short-term solution shipped in
hotwired/turbo-rails#248, this commit attempts to make the
test/turbo_test.rb file's setup consistent with the test harness setup
generated by Rails' engine generator code.

To that end, this commit:

• renames the test/turbo_test.rb file to test/test_helper.rb
• omits one-off require calls for particular dependencies
• re-orders the require calls so that the
  ../test/dummy/config/environment file is required ahead of the
  rails/test_help file

[seanpdoyle]

Unfortunately, this didn't resolve the CI failures.

My hunch is that there's a missing or misconfigured Active Record value somewhere that's resulting in records persisting across test runs.

[seanpdoyle]

Closing in favor of unifying streams of work into #327.