test-ui: Fix crasher on exit
test-ui: Fix crasher on exit

The test UI was crashing because we left closing through the
"Close" button destroy the window, which left all the pointers
dangling.

https://bugzilla.gnome.org/show_bug.cgi?id=724658

MathieuDuponchelle/grilo,grilofw/grilo,jasuarez/grilo,MathieuDuponchelle/grilo,MathieuDuponchelle/grilo,GNOME/grilo,GNOME/grilo,grilofw/grilo,jasuarez/grilo,grilofw/grilo,jasuarez/grilo