gmenumodel test: Wait for the expected events to happen

Previously, we waited an arbitrary 100ms or 200ms and then asserted
that the events had happened, but that might fail if the machine is
slow or heavily loaded.

We still wait for an arbitrary time for negative tests (asserting
that no more signals are received) because we don't have any way
to do better here.

Signed-off-by: Simon McVittie <smcv@collabora.com>
Bug-Debian: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=884661

https://bugzilla.gnome.org/show_bug.cgi?id=791744