tests/test-client: Log to stderr

The test-client reports command results to stdout, so make sure to log
e.g. debug messages to stderr, so they don't interfere with the result
test-runner reads.

Closes: https://gitlab.gnome.org/GNOME/mutter/-/issues/450
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2095>

src/tests/test-client.c

@@ -935,6 +935,8 @@ main(int argc, char **argv)
   GtkCssProvider *provider;
   GError *error = NULL;
 
+  g_log_writer_default_set_use_stderr (TRUE);
+
   g_option_context_add_main_entries (context, options, NULL);
 
   if (!g_option_context_parse (context,