nodejs: Add a test for callback garbage collection

[tronical]

Replaces #3772
Closes #3706

[ogoffart]

So there was no leak at all after all?

[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.

[ogoffart]

What if you add something like this:

Suggested change

	function scope() {
	let copy = demo;
	copy.say_hello = () => {
	console.log(copy.check);
	};
	}
	scope();

i.e: add another callback that use a property from some other variable with a small scope.

[tronical]

That did the trick, good idea. Pushed, now the test fails.

[tronical]

Stupid bug here: The instance is never created! The my_callback assignments happen to work because well, it's just a JS object. And because we don't have type safety, the compiler didn't warn us. So the test never really tested what it should have: instantiate the Rust code that presumably keeps a strong ref.

[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).