Sleep a bit less in notification tests (to fix KDE postinst)

In F28 tests, the notification 'counter' thing that we rely on
to check there's only *one* notification seems to suddenly
disappear...right around 10 minutes after the desktop starts up,
which is just how long our test idles for to catch crashes that
happen a little after boot. That causes test fails. Let's try
just cutting the wait down to 8 minutes to see if that helps.

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

tests/desktop_notifications.pm

@@ -69,7 +69,7 @@ sub run {
     # now, WE WAIT. this is just an unconditional wait - rather than
     # breaking if we see an update notification appear - so we catch
     # things that crash a few minutes after startup, etc.
-    for my $n (1..20) {
+    for my $n (1..16) {
         sleep 30;
         mouse_set 10, 10;
         mouse_hide;