Test coverage is also extended in this commit to:
Ensure that another transaction in the mempool is not evicted
when other transactions expire. Note: this other transaction does
not have any ancestors in the mempool that expired. Otherwise it
would be evicted from the mempool, and we already test this case.
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.