audit(4): Add tests for the fw class of syscalls.

truncate and ftruncate are the only syscalls in this class, apart from
certain variations of open and openat, which will be handled in a different
file.

Submitted by: aniketp
MFC after: 2 weeks
Sponsored by: Google, Inc. (GSoC 2018)
Differential Revision: https://reviews.freebsd.org/D15640