tests: make GtkMenu test more flexible
tests: make GtkMenu test more flexible

We were using gtk_menu_item_get_label() from a testcase to determine the
label associated with a menu item.  Future changes to GtkModelMenuItem
will cause this to stop working, so try a bit harder to find a label
inside of the item.

Lyude/gtk-,chergert/gtk,jigpu/gtk,bratsche/gtk-,Sidnioulz/SandboxGtk,ebassi/gtk,ahodesuka/gtk,jigpu/gtk,msteinert/gtk,davidgumberg/gtk,jessevdk/gtk,jessevdk/gtk,jessevdk/gtk,ebassi/gtk,Adamovskiy/gtk,msteinert/gtk,jadahl/gtk,grubersjoe/adwaita,Adamovskiy/gtk,jigpu/gtk,Lyude/gtk-,chergert/gtk,Lyude/gtk-,bratsche/gtk-,msteinert/gtk,Lyude/gtk-,Adamovskiy/gtk,msteinert/gtk,Adamovskiy/gtk,jigpu/gtk,grubersjoe/adwaita,jessevdk/gtk,chergert/gtk,Sidnioulz/SandboxGtk,grubersjoe/adwaita,ebassi/gtk,ahodesuka/gtk,jessevdk/gtk,ahodesuka/gtk,chergert/gtk,davidgumberg/gtk,alexlarsson/gtk,jigpu/gtk,Adamovskiy/gtk,ebassi/gtk,jadahl/gtk,davidgumberg/gtk,alexlarsson/gtk,grubersjoe/adwaita,alexlarsson/gtk,Distrotech/gtk2,ebassi/gtk,ahodesuka/gtk,alexlarsson/gtk,jadahl/gtk,bratsche/gtk-,jigpu/gtk,Sidnioulz/SandboxGtk,Lyude/gtk-,alexlarsson/gtk,chergert/gtk,Adamovskiy/gtk,Distrotech/gtk2,Sidnioulz/SandboxGtk,msteinert/gtk,bratsche/gtk-,Distrotech/gtk2,Lyude/gtk-,davidgumberg/gtk,Distrotech/gtk2,Distrotech/gtk2,grubersjoe/adwaita,ebassi/gtk,jadahl/gtk,jessevdk/gtk,jadahl/gtk,Sidnioulz/SandboxGtk,davidgumberg/gtk,msteinert/gtk,grubersjoe/adwaita,jigpu/gtk,ahodesuka/gtk,jadahl/gtk,chergert/gtk,alexlarsson/gtk,bratsche/gtk-,jigpu/gtk,Sidnioulz/SandboxGtk,Adamovskiy/gtk,alexlarsson/gtk,chergert/gtk,Lyude/gtk-,bratsche/gtk-,jadahl/gtk,ahodesuka/gtk,davidgumberg/gtk,grubersjoe/adwaita,ahodesuka/gtk,grubersjoe/adwaita,Distrotech/gtk2,davidgumberg/gtk,ahodesuka/gtk,davidgumberg/gtk,Lyude/gtk-,Adamovskiy/gtk,jessevdk/gtk,chergert/gtk,jadahl/gtk,alexlarsson/gtk