test-theme: Check return value of chdir()

I noticed the following warning while building the .deb package for
gnome-shell on Endless OS:

      ../src/st/test-theme.c: In function ‘main’:
      ../src/st/test-theme.c:549:3: warning: ignoring return value of ‘chdir’, declared with attribute warn_unused_result [-Wunused-result]
         chdir (cwd);
         ^~~~~~~~~~~

(Of course this is very unlikely to fail in practice.)

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1208

src/st/test-theme.c

@@ -546,7 +546,8 @@ main (int argc, char **argv)
 
   meta_test_init ();
 
-  chdir (cwd);
+  if (chdir (cwd) < 0)
+    g_error ("chdir('%s') failed: %s", cwd, g_strerror (errno));
 
   /* Make sure our assumptions about resolution are correct */
   g_object_set (clutter_settings_get_default (), "font-dpi", -1, NULL);