nodejs: Add a test for callback garbage collection

[tronical]

Replaces #3772 Closes #3706

[tronical]

Yes, at least I couldn't find it. But what's weird is that I don't quite understand how / why it works. But if the test looks sane to you, too, then that's the best thing we have for now to verify that we're not leaking.

[ogoffart]

Is there a test that doesn't do demo = null? Because that's not what i'd expect in real code. I'd use demo.hide() and let demo go out of scope.

[tronical]

Note to future self:

1. Show
2. GC should not collect
3. Hide
4. GC should collect

[tronical]

Rebasing this first and then trying to find out why it passes in the CI (but fails, as expected locally).