appiumtests: stop matching push_button because it's renamed
Reason for the change
https://github.com/GNOME/at-spi2-core/commit/0c2ebfad9ebf92913a326143345b841d54fa1866 and https://github.com/GNOME/at-spi2-core/commit/c83ef490a1fae350335737fe4797310625f6d2ce renamed "push button" to "button".