core: Add uniqueness test to AuditLogValue

Since the audit log value is supposed to be unique, this test will enforce
the uniqueness of the audit log values

Change-Id: I8e3f58d7ccf8a6477fafbac94d6e1057ecefe23a
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1145977
Signed-off-by: Tal Nisan <tnisan@redhat.com>