audit(4): add tests for extattr_get_file(2) and friends

This commit includes extattr_{get_file, get_fd, get_link, list_file,
list_fd, list_link}. It does not include any syscalls that modify, set, or
delete extended attributes, as those are in a different audit class.

Submitted by: aniketpt
MFC after: 2 weeks
Sponsored by: Google, Inc. (GSoC 2018)
Differential Revision: https://reviews.freebsd.org/D15859