tests/sys/audit: Skip extattr tests if extattrs are not supported

In the CheriBSD CI, we run the testsuite with /tmp as tmpfs. This causes
the extattr audit tests to fail since tmpfs does not (yet) support
extattrs. Skip those tests if the target path is on a file system that
does not support extended file attributes.

While touching these two files also convert the ATF_REQUIRE_EQ(-1, ...)
checks to use ATF_REQURIE_ERRNO().

Reviewed By: asomers
Differential Revision: https://reviews.freebsd.org/D28392