add: test that transactions expire from mempool

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.

test/functional/test_runner.py

@@ -175,6 +175,7 @@ BASE_SCRIPTS = [
     'wallet_balance.py',
     'feature_nulldummy.py',
     'mempool_accept.py',
+    'mempool_expiry.py',
     'wallet_import_rescan.py',
     'wallet_import_with_label.py',
     'rpc_bind.py --ipv4',