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>