Rather than sleeping and blocking everything, use a timeout source on
the main context so that inputs can continue to be handled while waiting
for a timeout.
This introduces no functional changes to the test.
Signed-off-by: Philip Withnall <pwithnall@gnome.org>
Helps: #2887
It should be doing already (or the tests would never work), but make it
clearer in the code that the same `GMainContext` is being iterated as is
being woken up in the rest of the test.
Signed-off-by: Philip Withnall <pwithnall@gnome.org>
Helps: #2887
