audit(4): Fix file descriptor leaks in ATF tests

Submitted by: aniketp
Reported by: Coverity
CID: 1393343 1393346 1392695 1392781 1391709 1392078 1392413
CID: 1392014 1392521 1393344 1393345 1393347 1393348 1393349
CID: 1393354 1393355 1393356 1393357 1393358 1393360 1393362
CID: 1393368 1393369 1393370 1393371 1393372 1393373 1393376
CID: 1393380 1393384 1393387 1393388 1393389
MFC after: 2 weeks
Sponsored by: Google, Inc (GSoC 2018)
Differential Revision: https://reviews.freebsd.org/D15782