b2ff500fb3 test: add "diamond" unit test to MempoolAncestryTests (Sebastian Falbesoner)
Pull request description:
Approaches #17271 (_Missing Unit Test for Ancestors "diamond"_).
If ancestors are represented more than once (in this case `ta` and `tb`), check that those are not overcounted.
ACKs for top commit:
laanwj:
ACK b2ff500fb3
Tree-SHA512: 82a6573cc7f0e82bf6fcfe207d7ddecbf297d2a203d22e95b73d887e3cb280f45a3c5f649161561c1be1eb560ff81b9b385868f205d1c12284211c2377e5ad99