tests: Iterate mainloop during launch test

When launching an application, we wait for the DBus response from
systemd before executing the binary. Because of this the main loop needs
to be iterated for spawning to completed and the file to be created.

Without this the test will time out if GLib was able to connect to the
session bus.

gio/tests/desktop-app-info.c

@@ -334,6 +334,7 @@ wait_for_file (const gchar *want_this,
    */
   while (access (want_this, F_OK) != 0)
     {
+      g_main_context_iteration (NULL, FALSE);
       g_usleep (100000); /* 100ms */
       g_assert_cmpuint (retries, >, 0);
       retries--;