Wait for screen change in desktop_terminal before typing pw

Sigh I hate this test. We seem to be typing root pw before the
terminal is ready for us:
https://openqa.fedoraproject.org/tests/745007#step/desktop_terminal/3
Let's try this. Hopefully it'll wait for the Password: prompt
before typing, without us having to actually add a needle...

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

tests/desktop_terminal.pm

@@ -11,7 +11,7 @@ sub run {
     wait_still_screen 5;
     # need to be root
     my $rootpass = get_var("ROOT_PASSWORD", "weakpassword");
-    type_string "su\n", 20;
+    wait_screen_change { type_string "su\n", 20; };
     wait_still_screen 3;
     # can't use type_safely for now as current implementation relies
     # on screen change checks, and there is no screen change here