While developing the lx brand on illumos/SmartOS, ltp was helpful. It may not be complete, but it is a pretty good start.

https://linux-test-project.readthedocs.io/en/latest/

LTP really needs to be a part of Linux itself.

Here is a list of implemented syscalls, but of course each checked one could still be slightly incompatible:

https://asterinas.github.io/book/kernel/linux-compatibility....

There's also tons of ioctls and /proc and what not.