Address fixture path deprecation warnings

[cjlarose]

Fixes deprecation warning in Rails 7.1

DEPRECATION WARNING: TestFixtures#fixture_path is deprecated and will be removed in Rails 7.2. Use #fixture_paths instead.
If multiple fixture paths have been configured with #fixture_paths, then #fixture_path will just return
the first path.

Since the test suite supports older Rails versions and fixtures_paths was introduced in Rails 7.1, we continue to call fixture_path unless fixtures_path is available.