tests: Remove spurious gtk_container_add()
Commit bd71e744 removed gtk_box_pack_end(), but it added a gtk_container_add() with an uninitialised widget, and the compiler is very unhappy about it.
Commit bd71e744 removed gtk_box_pack_end(), but it added a gtk_container_add() with an uninitialised widget, and the compiler is very unhappy about it.