Improve LastCommonAncestor performance + add tests

[sipa]

In theory, the LastCommonAncestor function in chain.cpp can take $\mathcal{O}(n)$ time, walking over the entire chain, if the forking point is very early, which could take ~milliseconds. I expect this to be very rare in normal occurrences, but it seems nontrivial to reason about worst cases as it's accessible from several places in net_processing.

This PR modifies the algorithm to make use of the CBlockIndex::pskip skip pointers to find the forking point in sublinear time (a simulation shows that for heights up to $34 \cdot 4^k - 2$ and $k \geq 8$, no more than $k^2 + 10k + 13$ steps are ever needed), in a way that should be nearly free - at worst the same number of memory accesses should be made, with a tiny increase in computation.

As it appears we didn't really have tests for this function, unit tests are added for that function as well as CBlockIndex::GetAncestor().

This is inspired by #32180 (comment)