core: AuditLogDAOTest date format

Since SimpleDateFormat is inherently not thread-safe and some runners
may run different tests in different thread, it's a potential bug to use
a static instance of it.

Additionally, this issue is picked up by recent version of FindBugs and
Coverity, generating noise.

Change-Id: I41384f1f061bcffbc08c2f0fda8633d2f42bd5a9
Signed-off-by: Allon Mureinik <amureini@redhat.com>