audit(4): add tests for open(2) and openat(2)

These syscalls are atypical, because each one corresponds to several
different audit events, and they each pass several different audit class
filters.

Submitted by: aniketp
MFC after: 2 weeks
Sponsored by: Google, Inc. (GSoC 2018)
Differential Revision: https://reviews.freebsd.org/D15657