test(plugins): wait on the stream's producer instead of on the clock

[datlechin]

Follow-up to #2972, which fixed the flake by polling. This replaces the polling with the producer saying when it is done.

holdingTheStreamKeepsItAlive asserts that a stream nobody is reading, but which is still held in scope, is not aborted: its producer runs all 20 rows. The producer paces itself at 20ms a row, so the original fixed sleep asserted the runner's load as much as the stream's behaviour, and CI saw 19 of 20. #2972 polled the counter instead, which is not a clock but is still a guess about how long to keep asking.

The producer now signals once its loop ends and the test awaits that signal. An abort would leave the loop early and still signal, so the count is what decides, which is the same assertion as before. .timeLimit(.minutes(1)) is what fails a producer that never finishes, rather than the suite hanging.

The suite passes three runs in a row locally.