test: add "diamond" unit test to MempoolAncestryTests

Summary:
Approaches #17271.
If ancestors are represented more than once, check that those are not
overcounted.

This is a backport of Core PR17363

Depends on D4915

Test Plan:

make check

Reviewers: #bitcoin_abc, markblundeberg

Reviewed By: #bitcoin_abc, markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D4916