audit(4): add tests for the fd audit class

The only syscalls in this class are rmdir, unlink, unlinkat, rename, and
renameat. Also, set is_exclusive for all audit(4) tests, because they can
start and stop auditd.

Submitted by: aniketp
MFC after: 2 weeks
Sponsored by: Google, Inc. (GSoC 2018)
Differential Revision: https://reviews.freebsd.org/D15647