Fix Rails 5.2-8.1 compatibility with minimal changes

- Keep original test_helper.rb structure
- Add SimpleCov improvements (filters, groups, branch coverage)
- Add Rails 7.1+ initialization order support (define and initialize TestApp before rails/test_help)
- Keep Rails < 7.1 compatibility (define TestApp after rails/test_help)
- Add Rails 7.2+ deprecation warning handling
- Add Rails 7.2+ fixture_paths= support
- Add Rails 8.0+ schema_format compatibility

Author: takaokouji