autopkgtest: Run python3 test with xvfb to bypass warning with GTK 4.10.2
debian/tests/run-with-display copied from GTK3/GTK4 packaging
Fixes https://autopkgtest.ubuntu.com/packages/liba/libadwaita-1/mantic/amd64
Triggered by https://gitlab.gnome.org/GNOME/gtk/-/commit/61536ea1a