tests: Allow a potential error to be shown before failing
tests: Allow a potential error to be shown before failing

When the test fails, let's first give it the chance to print the error
message in order to help debugging failures.

https://bugzilla.gnome.org/show_bug.cgi?id=786259

GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf