test: add "diamond" unit test to MempoolAncestryTests
test: add "diamond" unit test to MempoolAncestryTests

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


Former-commit-id: a41405753b72ba68ae5a55d82df634cff1ad75e2
syscoin/syscoin2,syscoin/syscoin,syscoin/syscoin2,syscoin/syscoin,syscoin/syscoin2,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin2,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin2,syscoin/syscoin2,syscoin/syscoin,syscoin/syscoin