test: Verify findCommonAncestor always initializes outputs

Summary:

Also add code comment to clarify surprising code noted by practicalswift
https://github.com/bitcoin/bitcoin/pull/18657#issuecomment-614278450

See also discussion on unmerged PR18657, for why this test is added.

This is a backport of Core PR18660

Test Plan: ninja && ninja check

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8932