Prefer make_awaitable over defer.succeed in tests (#12505)

When configuring the return values of mocks, prefer awaitables from
`make_awaitable` over `defer.succeed`. `Deferred`s are only awaitable
once, so it is inappropriate for a mock to return the same `Deferred`
multiple times.

Also update `run_in_background` to support functions that return
arbitrary awaitables.

Signed-off-by: Sean Quah <seanq@element.io>

tests/federation/test_federation_client.py

@@ -83,7 +83,7 @@ class FederationClientTest(FederatingHomeserverTestCase):
         )
 
         # mock up the response, and have the agent return it
-        self._mock_agent.request.return_value = defer.succeed(
+        self._mock_agent.request.side_effect = lambda *args, **kwargs: defer.succeed(
             _mock_response(
                 {
                     "pdus": [