GTK tests: Add and use context manager for realized widgets
GTK tests: Add and use context manager for realized widgets

https://bugzilla.gnome.org/show_bug.cgi?id=693377

GNOME/pygobject,GNOME/pygobject,davibe/pygobject,pexip/pygobject,pexip/pygobject,davibe/pygobject,GNOME/pygobject,sfeltman/pygobject,davibe/pygobject,davibe/pygobject,sfeltman/pygobject,sfeltman/pygobject,pexip/pygobject