Testing: Work out why some unit tests cannot run independently

**Describe the bug**
The test `test_update_include_location_in_file_as_list` in `test_nexus_file_class.py` currently fails if run by itself rather than as part of the overall test suite.

**To Reproduce**
Steps to reproduce the behavior:
1. Run the test `test_update_include_location_in_file_as_list` in `test_nexus_file_class.py` on it's own.

**Expected behavior**
The test passes.

**Additional context**
This is likely because of a mock set elsewhere that this test relies upon. That mock should be either set in a fixture that this test uses, or in a localised conftest file.