add a short wait_still_screen to _graphical_wait_login

looks like we had a good ol' 'sliding-in-from-the-side' fail
in https://openqa.fedoraproject.org/tests/32163 .

tests/_graphical_wait_login.pm

@@ -31,6 +31,8 @@ sub run {
         # is set in which case it will have been done already
         if (get_var("DESKTOP") eq 'gnome' && !get_var("START_AFTER_TEST")) {
             assert_screen "next_button", 60;
+            # wait a sec in case of animation
+            wait_still_screen 1;
             for my $n (1..3) {
                 # click 'Next' three times, moving the mouse to avoid
                 # highlight problems, sleeping to give it time to get