tests: More robust tree view realization
tests: More robust tree view realization

With current GNOME git head, the Gtk.TreeViews were not realized enough any
more to receive property values. Put them into a Gtk.Dialog now and show it to
ensure that they are realized.

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