Make audit tests depend on /dev/auditpipe. This should fix
some 416 failing tests on armv7:

https://ci.freebsd.org/job/FreeBSD-head-armv7-test/lastCompletedBuild/testReport/

Reviewed by: asomers
MFC after: 2 weeks
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D25144