Suppress libselinux logging during CTS tests.

Otherwise we get a large number of avc:  denied messages during
SELinux testing that are not useful in diagnosing any actual CTS
failures.  The test itself already logs the actual failures.

Change-Id: Iaa93abcb0913860649cdeb3e6a44af271b44ee3c
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>