Add tests for WeakReference, Finalizer and NativeFinalizer

[mraleph]

No description provided.

[mraleph]

Likely we will have to create VM only tests and rely on JS implementation to be correct.

[dcharkes]

@mraleph I have landed tests as part of my CLs, and co19 has also landed tests. Should this issue be closed?

@rileyporter is the JS side covered with tests as well?

[rileyporter]

For the JS side, yes, I think we can close this issue.

We added some minimal tests for the JS API, but we aren't testing the native JS behavior that is underlying the web implementation. The garbage collection behavior for JS is not guaranteed or reliably controlled, so any tests relying on the JS behavior would probably be flaky and not give us much extra coverage. There might be some extra coverage coming from the co19 tests, but I think they ran into the same issues with testing the JS behavior.