appiumtests: downgrade test windows to GTK3
For some reason (https://gitlab.gnome.org/GNOME/gtk/-/issues/6942) GTK4 always triggers the assertion.
Gdk:ERROR:../gdk/gdkmemoryformat.c:1552:gdk_memory_depth_merge: assertion failed: (depth2 < GDK_N_DEPTHS)
Bail out! Gdk:ERROR:../gdk/gdkmemoryformat.c:1552:gdk_memory_depth_merge: assertion failed: (depth2 < GDK_N_DEPTHS)
Edited by Fushan Wen