appmonitor test: avoid /usr/share

Set XDG_DATA_DIRS to make sure we don't use /usr/share from the
appmonitor test.  We will soon throw a warning if we find defaults.list,
so make sure we don't open ourselves up to that if there is one on the
system.

https://bugzilla.gnome.org/show_bug.cgi?id=728040

gio/tests/appmonitor.c

@@ -104,6 +104,7 @@ main (int argc, char *argv[])
   gchar *path;
 
   path = g_mkdtemp (g_strdup ("app_monitor_XXXXXX"));
+  g_setenv ("XDG_DATA_DIRS", path, TRUE);
   g_setenv ("XDG_DATA_HOME", path, TRUE);
 
   g_test_init (&argc, &argv, NULL);