Add a keypress to the 'keepalive' loop in desktop_notifications

Just repositioning the mouse appears not to be enough to prevent
the sesssion going idle any more, since the 20200731.n.0 compose.
Not sure what causes this, probably the kernel. Adding a space
keypress seems to help in both KDE and GNOME.

Signed-off-by: Adam Williamson <awilliam@redhat.com>

tests/desktop_notifications.pm

@@ -74,6 +74,7 @@ sub run {
     for my $n (1..16) {
         sleep 30;
         mouse_set 10, 10;
+        send_key "spc";
         mouse_hide;
     }
     if ($desktop eq 'gnome') {