audit(4): add tests for stat(2) and friends

This revision adds auditability tests for stat, lstat, fstat, and fstatat,
all from the fa audit class. More tests from that audit class will follow.

Submitted by: aniketp
MFC after: 2 weeks
Sponsored by: Google, Inc. (GSoC 2018)
Differential Revision: https://reviews.freebsd.org/D15709