gmenumodel test: If something goes wrong, don't wait forever

I'm about to add some loops that would otherwise wait indefinitely.

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