refactor(test): remove unused test files

- Removed test files for routes and middleware that are no longer in use
- Deleted test/routes/api/v1/countries/index.dart
- Deleted test/routes/api/v1/countries/[isoCode].dart
- Deleted test/routes/api/v1/index_test.dart
- Deleted test/src/middleware/error_handler_test.dart

Author: fulleni