Added a `worker` property to access the underlying worker

[aelgasser]

As discussed in https://github.com/GoogleChromeLabs/comlink/pull/487, this is a proposal to implement a worker property to the wrapper returned by this loader. This allows the user to properly terminate the Worker.

I also fixed the setup and tear down code in tests as the spy put in place of the original worker was kept by from one test suite to another and made further tests fail.