core: TestAuditLoggableBase serial warning

Suppressed the serial warning on TestAuditLoggableBase. It is just a
test class, and will never be serialized or deserialized.

Change-Id: Ic88229b7c147bbcd59d6c88a924d81603dcf18b3
Signed-off-by: Allon Mureinik <amureini@redhat.com>