Timeout on flush in testing (#16864)

Timeout on flush in testing (#16864)

* Timeout on flush in testing

At the end of each test the queues are flushed. At present there is no limit on the
length of time a flush can take which can lead to long flushes.

However, if the CI task is cancelled we lose the log information as to where the long
flush was taking place.

This PR simply adds a default time limit of 2 minutes - at which point an error will
be produced. This should allow us to more easily find the culprit.

Signed-off-by: Andrew Thornton <art27@cantab.net>
* return better error

Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: 6543 <6543@obermui.de>