tests: Define a primary key
tests: Define a primary key

Avoids the following warning:

  SAWarning: Could not assemble any primary keys for locally mapped
  table 'fake_table_inh' - no rows will be persisted in this Table.
    class FakeTableJoinedInh(FakeTable):

Change-Id: Ia7795be05e6364d8b2d70be70f5154094a55161a
Signed-off-by: Stephen Finucane <4725dc7f3632bf126d9fbdc30c347e91e8d2fe16@redhat.com>

openstack/oslo.db,openstack/oslo.db