FS unit test fixes for UNC paths

[mwichmann]

Managed to fail a number of UNC path tests by moving everything, even the %TEMP% directory to a dev drive from the C drive. A structuring problem meant some paths weren't trimmed of trailing slashes as they were intended to - we just do a string comparison in the cases, not a path-aware comparison, so this was enough for an error. Resynced three similar tests, added some docstrings and comments, etc.

No doc impacts (except docstrings).

Contributor Checklist:

• [X] I have created a new test or updated the unit tests to cover the new/changed functionality.
• [X] I have updated CHANGES.txt and RELEASE.txt (and read the README.rst).
• [X] I have updated the appropriate documentation