Stress testing one of the notification overlay tests by running it 10000
times on repeat has shown that it is susceptible to intermittent
failures due to races between delays and asserts checking the number of
currently progressing notifications and the actual progress update,
which contains a random generation factor.
Replace step sequences checking for notification completion by waiting
and asserting with explicit until steps that don't terminate unless
there are zero progressing notifications.
fd8c5d7706