Avoid update notifications in start-stop tests.

Sometimes, update notification would pop up during the testing
and prevent the needles from being matched correctly.
The addition to the code makes the notifications go away for updates.

tests/apps_gnome_preset.pm

@@ -10,6 +10,8 @@ sub run {
     my $self = shift;
     #  Change the background to black.
     solidify_wallpaper;
+    # Surpress the update notification
+    set_update_notification_timestamp();
 }
 
 # If this test fails, the others will probably start failing too,