audit(4): add tests for extattr_set_file and friends

Includes extattr_{set_file, _set_fd, _set_link, _delete_file, _delete_fd,
_delete_link}

Submitted by: aniketp
MFC after: 2 weeks
Sponsored by: Google, Inc. (GSoC 2018)
Differential Revision: https://reviews.freebsd.org/D15867