uexterr_gettext: add tests

[asomers]

Summary

Add tests for the new extended errno feature, using a custom kernel
module to report errors to userland.

Sponsored by: ConnectWise
MFC after: 2 weeks

Abandon the custom kernel module. Rely on existing syscalls instead.

move the test into the kern directory

Now that the custom kld is gone, it doesn't need its own directory.

[kib]

sys/kern/sys_generic.c

2316

Add FALLTHROUGH annotation?

tests/sys/kern/exterr_test.c

54

I never intended for the returned strings to be part of the ABI.

[asomers]

tests/sys/kern/exterr_test.c

54

Yeah, I didn't want to assume that the strings would never change. That's why I created a custom kernel module, at first. Would you like some other assertion instead of this?

[kib]

tests/sys/kern/exterr_test.c

54

Might be a simple approach is to add a comment that the string in the test might need to be updated if kernel changed, and this is not a functional failure then.