audit(4): add tests for the process-control audit class

Tested syscalls include rfork(2), chdir(2), fchdir(2), chroot(2),
getresuid(2), getresgid(2), setpriority(2), setgroups(2), setpgrp(2),
setrlimit(2), setlogin(2), mlock(2), munlock(2), minherit(2), rtprio(2),
profil(2), ktrace(2), ptrace(2), fork(2), umask(2), setuid(2), setgid(2),
seteuid(2), and setegid(2). The last six are only tested in the success
case, either because they're infalliable or a failure is difficult to cause
on-demand.

Submitted by: aniketp
MFC after: 2 weeks
Sponsored by: Google, Inc. (GSoC 2018)
Differential Revision: https://reviews.freebsd.org/D15966