Enhance tests for assignment create_grant when no user or group

There were no tests that show the behavior of create_grant when
the user or group doesn't exist. Turns out the behavior is
different depending on the backend (sql vs kvs vs ldap).

Change-Id: I71c3e2b94aaa44e40c8a891b49cc2a9961cee16f
Related-Bug: #1239476
Merge "Enhance tests for assignment create_grant when no user or group"