Fix CI mock filesystem fixtures
This PR fixes the fixtures of our CI mock filesystems.

Before, we had to pass `clobber=True` to `fsspec.register_implementation` to overwrite the still present previously added "mock" filesystem. That meant that the mock filesystem fixture was not working properly, because the previously added "mock" filesystem, should have been deleted by the fixture.

This PR fixes the mock filesystem fixtures, so that the "mock" filesystem is properly deleted from the inner `fsspec` registry.

Tests were added to check the correct behavior of the mock filesystem fixtures.

Related to:
- #5733