So the cleanup test is not passing, however some of the temporary files/directories that are shown in the diff are not created by us; I have added a print before the cleanup to see what files are to be cleaned up (except if I am missing something).

Maybe these temp files are created by mergerfs? I wouldn't consider this impossible. That brings me the following thought. Why do we even want to delete the temporary log/auxilliary files? They are useful for debugging, and they are tiny. I think the only thing we actually want to delete is any left state from overlayfs, any mounts that we did, and the upperdir (which could get big). Everything else doesn't need to be deleted. Based on that, I think we should properly investigate what state does overlayfs leave behind and discuss how to solve this issue in #93 and only merge the testing overhaul from this (#92) PR.

@mgree what do you think about my comment above? Is it OK if we merge the testing overhaul from here and leave cleanup for a proper PR in the future after 0.2 (after we properly understand what state overlayfs leaves behind).

Yes, I think that's fine.