tests: mark tests skipped due to no display as skipped

We were skipping tests when there is no display, which is always the
case on Windows. However, these tests always showed as succeeded.
Unless you looked at the test log, you wouldn't noticed that most of
the tests weren't actually performed.

Exit these tests with the value GIMP_EXIT_TEST_SKIPPED instead of
EXIT_SUCCESS.

app/tests/tests.c

@@ -278,6 +278,6 @@ gimp_test_bail_if_no_display (void)
   if (! g_getenv ("DISPLAY"))
     {
       g_message ("No DISPLAY set, not running UI tests\n");
-      exit (EXIT_SUCCESS);
+      exit (GIMP_EXIT_TEST_SKIPPED);
     }
 }