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>