In 3.8 a lot of asyncio functions have their `loop` parameter
deprecated. So we change the semantics of uvloop tests to never
pass the loop explicitly (unless to Future objects, when necessary)
That means that we now also need to set up asyncio/uvloop loop policies
for tests in setUp hooks.
uvloop should consistently pass around the loop reference in its
internal APIs.
Even though explicit passing of event loop objects is discouraged
in asyncio programs, uvloop itself is too low-level to rely on
get_event_loop.