Set a default timeout for sleep_until_queue_empty

During acceptance test writing, this method may spin forever if the queue
doesn't clear properly (due to a fault often in the authors code most
probably).

To stop this from happening, we should set a reasonable default for this,
as we should constrain all tests from infinitely looping if we can. While
the parameter can be overriden at usage time, having a default will mean
infinite loops don't occur by accidental misusage.

Signed-off-by: Ken Barber <ken@bob.sh>