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` and `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 partially reverts commit c6ddf18, "Call uv_async_init lazily to avoid keeping process alive."