The timeout we're using here is 0.5s (500ms), but the actual time taken
is unbounded, because the OS scheduler might not schedule our process
for an arbitrary length of time after we become runnable.

We previously allowed up to 1 second, but in the CI jobs for !9
and !18

 we've seen this take up to 3.4 seconds (presumably
because other tests, or other jobs running on the same shared
infrastructure, starved this process). Allow up to 10 seconds to guard
against spurious failures.

The timeout used in the production system.conf is 150 seconds (2½
minutes), and we're only using the shorter 500ms timeout here to make
the test complete more quickly, so ±10 seconds is relatively
insignificant: the main thing is that it's finite.

Signed-off-by: Simon McVittie <smcv@collabora.com>
(cherry picked from commit 20e6eb7c)