audit(4): add more test cases for auditon(2)

auditon(2) is an ioctl-like syscall with several different variants, each of
which has a distinct audit event. This commit tests the remaining variants
that weren't tested in r336564.

Submitted by: aniketp
MFC after: 2 weeks
X-MFC-With: 336564
Sponsored by: Google, Inc. (GSoC 2018)
Differential Revision: https://reviews.freebsd.org/D16381