Issue #3615122: Update test module package metadata

Changes test modules to package: Testing:

• core/modules/jsonapi/tests/modules/jsonapi_response_validator/jsonapi_response_validator.info.yml
• core/modules/system/tests/modules/legacy_hook_test/legacy_hook_test.info.yml

Adds a unit test that dynamically discovers non-hidden test modules beneath core test directories.

• Excludes fixture directories, hidden modules, and modules that deliberately use Core packages to test module metadata.
• Adds the missing Testing package to the existing user_route_alter_test module

Closes #3615122