test-theme: Force default resolution

Clutter no longer hard-codes a resolution of 96 DPI (although that's
still the default), so any assertions of sizes for physical units
may be off.

Fix this by setting up the test environment according to the
assumptions.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/419

src/st/test-theme.c

@@ -542,6 +542,9 @@ main (int argc, char **argv)
   if (clutter_init (&argc, &argv) != CLUTTER_INIT_SUCCESS)
     return 1;
 
+  /* Make sure our assumptions about resolution are correct */
+  g_object_set (clutter_settings_get_default (), "font-dpi", -1, NULL);
+
   file = g_file_new_for_path ("st/test-theme.css");
   theme = st_theme_new (file, NULL, NULL);
   g_object_unref (file);