core: AuditLogableBaseTest synthetic access

Changed private members to protected to solve synthetic-access warnings.
This patch contributes to the code cleanliness (less warnings) and may
also grant a (very) minor performance boost.

Change-Id: I9c238908055e3ee864567233dc145987bced8ba6
Signed-off-by: Allon Mureinik <amureini@redhat.com>