test: check custom descendant limit in mempool_packages.py

Summary:

To test the custom descendant limit on node1 (passed by the argument
-limitdescendantcount), we check for four conditions:

-> the # of txs in the node1 mempool is equal to the limit
   (plus 1 for the parent tx, plus the # txs from the previous ancestor
   test which are still in)
-> all txs in node1 mempool are a subset of txs in node0 mempool
-> part of the constructed descendant-chain (the first ones up to the
   limit) are contained in node1 mempool
-> the remaining part of the constructed descendant-chain (all after the
   first ones up to the limit) is *not* contained in node1 mempool

Note: Core accepts one more descendant than we do, as of PR15681 (change related to Lightning Network and bumping fees)

This is backport of Core PR17461

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

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8770