Fix path provider persistence issues in tests

While working on something else I noticed there were issues with persisted state for the global `PATH_PROVIDER`. I tried to add a cleanup for this but it is worth its own PR:

Something like this in `common/beamlines/beamline_utils` :

```
def clear_path_provider():
if "PATH_PROVIDER" in globals():
global PATH_PROVIDER
del PATH_PROVIDER
```
and a cleanup fixture in conftest.
```
@pytest.fixture(autouse=True)
def path_provider_cleanup():
yield
beamline_utils.clear_path_provider()
```

## Acceptance Criteria
* The tests don't rely on accidentally persisted state, and the setting of globals is explicit