Separate the audit(4) tests for auditon(2)'s individual commands

auditon(2) is an ioctl-like syscall with several different variants, each of
which has a distinct audit event. Write separate audit(4) tests for each
variant.

Submitted by: aniketp
MFC after: 2 weeks
Sponsored by: Google, Inc. (GSoC 2018)
Differential Revision: https://reviews.freebsd.org/D16255