if a gather whose futures are already all done, but didn't get the
chance to return yet gets cancelled, it still has to propagate that
cancellation up the task chain.

In the past the test tests a proxy to that, some behavior of a
_GatheringFuture that will lead to this propagation. This test now tests
the real thing: we test that the cancellation is propagated.

in the past, a special _GatheringFuture inherited from Future.

This is very archaic. Now we simply use a Task.

gather now needs an event loop round more to work, just give it that
time.