Fix get_diagnostics() test for no compute consumer

If we time out call()s in the fake RPC driver when there are no
consumers for a topic, the get_diagnostics() test fails when we attempt
to call() out to compute.

Mock up the get_diagnostics() rpcapi and verify it gets called.

blueprint: oslo-messaging
Change-Id: I4e33d089a9b01ea6dbe31ddf3c67c1733fce10aa
Merge "Fix get_diagnostics() test for no compute consumer"