When you test a file system are you testing it via the libc APIs, creating, deleting, checking filesystem objects and so on? Or is it done at a lower level API (that I’d hope is the same across filesystems, different features excepted)
For filesystems the former. Having test code running in kernel space is relatively uncommon, though some kernel subsystems do that.
"can't even properly test it anymore" almost certainly means not that it is impossible for the maintainer, but more like they have better things to spend their time on.
