D5417: rust: translated random test of missingancestors

[Yuya Nishihara]

>   This test is useful for me, because it's the only one that really tests the corectness of MissingAncestor, and it's not a bench either.
>   I agree it feels out of place in  with the unit tests, so my proposal is to make an integration test out of it.

or examples/ to build a dev-only standalone executable?

I don't know which is better, but I think we'll need a way to get around
the randomness (e.g. pass in a static random seed to reproduce a failed
example.)