audit(4) tests: require /etc/rc.d/auditd

These tests should be skipped if /etc/rc.d/auditd is missing, which could be
the case if world was built with WITHOUT_AUDIT set. Also, one test case
requires /etc/rc.d/accounting.

Submitted by: ngie
MFC after: 2 weeks
Pull Request: https://github.com/freebsd/freebsd/pull/240