testutils: Document what happens by default and how to change it
testutils: Document what happens by default and how to change it

Signed-off-by: Simon McVittie <smcv@collabora.com>

https://bugzilla.gnome.org/show_bug.cgi?id=791745

endlessm/glib,johne53/MB3Glib,johne53/MB3Glib,johne53/MB3Glib,endlessm/glib,endlessm/glib,johne53/MB3Glib,endlessm/glib,endlessm/glib,johne53/MB3Glib,johne53/MB3Glib