ZTS events_002: Improve speed and reliability

events_002 exercises the ZED, ensuring that it neither misses events,
nor reporting events twice.

On slow test hardware, some of the timeouts are insufficient to allow
the ZED to properly settle. Conversely, on fast hardware these same
timeouts are too long, unnecessarily slowing the test run.

Instead of using a fixed timeout, wait for the expected final event
before returning. Additionally, wait with a timeout for unexpected
events to avoid missing them if they show up late.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Antonio Russo <aerusso@aerusso.net>
Closes #11703