Rename `test_app_path` -> `path_to_test_app` (avoids minitest warning)

[mjankowski]

In rails edge there's a helper in place that looks for tests without assertions and warns - https://github.com/rails/rails/blob/main/activesupport/test/testing/test_without_assertions_test.rb#L20

The rails edge CI runs have such a warning, example: https://github.com/ElMassimo/vite_ruby/actions/runs/9424262054/job/25964175625#step:6:12

This change just renames the method to not start with test_ to silence that warning.