in gdbus-proxy.c we see this:

static void
test_async (void)
{
  g_dbus_proxy_new_for_bus (G_BUS_TYPE_SESSION,
                            G_DBUS_PROXY_FLAGS_NONE,
                            NULL,                      /* GDBusInterfaceInfo */
                            "com.example.TestService", /* name */
                            "/com/example/TestObject", /* object path */
                            "com.example.Frob",        /* interface */
                            NULL, /* GCancellable */
                            proxy_ready,
                            NULL);
}


No main loop ever runs and, as a result, proxy_ready() is never called.  That's particularly bad because proxy_ready() is actually failing (as can be seen if you add another test to the same file that runs the mainloop later on).