Call uv_unref(&g_async) to allow process to exit.

This is a better way of fixing #47, since it avoids the extra work of
calling uv_close and reuses existing libuv APIs (uv_{ref,unref}).

I also took the liberty of removing the --forceexit option from the test
command, now that g_async no longer prevents test processes from exiting.

This partially reverts commit c6ddf186333a25cf52b3ab7c093a6d7b930ec829,
"Call uv_async_init lazily to avoid keeping process alive."