Try to work around #1444225 with a sleep in disk_guided_empty

IIRC disk_guided_empty is the only storage test that clicks
through INSTALLATION DESTINATION *really* fast, so let's try
adding a 2-second sleep to it to see if it works around the
'sometimes spoke shows as incomplete' bug that cropped up in
F26 and hasn't been fixed yet and tends to cause failures.

tests/disk_guided_empty.pm

@@ -15,6 +15,8 @@ sub run {
         assert_screen "anaconda_install_destination_updates", 30;
     }
 
+    # try and workaround #1444225 by waiting a bit before clicking Done
+    sleep 2;
     assert_and_click "anaconda_spoke_done";
 
     # Anaconda hub