Refs #21831 -- Softened the TestClient dependency on contrib.auth.

This is to prevent an import of django.test causing an import (and thus
an implicit checks regisration) for an app that may not be in
`INSTALLED_APPS`.

Better fixes may be possible when #20915 and/or #21829 are addressed.

Thanks to @carljm for the report.
Fixes #21833 -- Fix UserSettingsHolder.is_overridden() and add tests.
Fix a check_framework test so it doesn't fail when TEST_RUNNER is set (e.g. on CI).
Removed some stray newlines which caused flake8 issues
Changed django-admin back to django-admin.py. Refs #21827
Fixed #21674 -- Deprecated the import_by_path() function in favor of import_string().