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

This is a better way of fixing #47, since it requires no additional state (e.g. `g_watch_count`), avoids the extra work of calling `uv_close(&g_async)`, and reuses an existing libuv API (`uv_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 reverts commit c6ddf186333a25cf52b3ab7c093a6d7b930ec829, "Call uv_async_init lazily to avoid keeping process alive."