Externally "testing" a filesystem isn't exactly easy either. If an error occurs, there's not gonna be an exception thrown that you can send off to telemetry.

If you're lucky, your customer notices that files are missing, understands that it has to be a bug in the operating system and maybe even has a rough idea what they were doing that caused the bug to occur, then calls up support and your first-level support is competent enough to direct the problem to the filesystem people and then it's still going to require a lot of luck for that department to reproduce the problem and to actually find out what in the code is causing it.

Apple authored a tool called fsx (file system exerciser), but I doubt it checks for corner cases for loopback mounts. Maybe they should add such tests.

That tool has been around for ages, I think it might originally be from SGI.

They say it takes ten years to get the bugs out of a filesystem. Unfortunately I think a general release is necessary to mature a new FS.