tests: Fix skipping of appinfo tests when DISPLAY is unset

Use g_test_skip() so that the TAP output is correct for the tests,
rather than printing using g_printerr().

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://gitlab.gnome.org/GNOME/glib/issues/640
diff --git a/gio/tests/appinfo.c b/gio/tests/appinfo.c
index ea4809d..a52bc70 100644
--- a/gio/tests/appinfo.c
+++ b/gio/tests/appinfo.c
@@ -17,7 +17,7 @@
 
   if (g_getenv ("DISPLAY") == NULL || g_getenv ("DISPLAY")[0] == '\0')
     {
-      g_printerr ("No DISPLAY.  Skipping test.  ");
+      g_test_skip ("No DISPLAY set");
       return;
     }