test: check custom ancestor limit in mempool_packages.py

Summary:

To test the custom ancestor limit on node1 (passed by the argument
-limitancestorcount), we check for three conditions:

-> the # of txs in the node1 mempool is equal to the the limit
-> all txs in node1 mempool are a subset of txs in node0 mempool
-> the node1 mempool txs match the start of the constructed tx-chain

This is a backport of Core PR17435

Test Plan: ninja && test/functional/test_runner.py mempool_packages.py

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, majcosta

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8262