Clean up testing to use use_settings decorator etc

Older tests are a mess, clean up to be much more mocked and not as integrate-y, also use decorators

```
37 def __init__(self, use_settings=None):
38 if use_settings:
39 # Used by testing, TODO remove
40 self.placeholders = use_settings.PLACEHOLDERS
```

Locate the older tests and the __init__ method containing the use_settings parameter and PLACEHOLDERS assignment. First inventory which tests depend on integration behavior, then review existing mocking and decorator patterns in the test suite. Done means the affected tests use the established decorator and mocking approach rather than the current integration-heavy setup.

Written by the indexing model from the issue text.