add: test that transactions expire from mempool

Summary:

This tests that a mempool transaction expires after a given timeout
and its children are removed as well.

Both the default expiry timeout defied by DEFAULT_MEMPOOL_EXPIRY and
a user definable expiry timeout via the -mempoolexpiry=<n> command
line argument (<n> is the timeout in hours) are tested.

This is a backport of Core PR18172

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

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8766