desktop_terminal: assert we actually see terminal app

...Apparently a wait_still_screen 5 isn't always enough on
aarch64:
https://openqa.stg.fedoraproject.org/tests/963387#step/desktop_terminal/2
so, this. Note that konsole_runs has apps_run_terminal tag, so
this won't break the test on KDE.

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

tests/desktop_terminal.pm

@@ -7,6 +7,7 @@ sub run {
     my $self=shift;
     check_desktop;
     menu_launch_type('terminal');
+    assert_screen "apps_run_terminal";
     wait_still_screen 5;
     # need to be root
     my $rootpass = get_var("ROOT_PASSWORD", "weakpassword");