tests: Attempt fixing intermittents in async notification tests
tests: Attempt fixing intermittents in async notification tests

`no_receiver_notification()`, and especially
`no_senders_notification_try_recv()`, fail quite frequently on CI.
The only reason I can think of is that apparently the kernel sometimes
doesn't process the close event immediately, but rather continues
running the user thread, so the next read or write call doesn't get the
notification yet.

Attempting to fix this by repeatedly polling for the notifications to
arrive. Let's hope this doesn't have any undesired side effects...

Note that `no_senders_notification()` should not be affected, since that
one isn't async. (The regular `recv()` method waits for a result to
arrive.)

servo/ipc-channel