test: Test that an unconfirmed not-in-mempool chain is rebroadcast

The test checks that parent txs are broadcast before child txs.

The previous behavior is that the rebroadcasting would simply iterate mapWallet. As
mapWallet is a std::unsorted_map, the child can sometimes come before the parent and thus
be rebroadcast in the wrong order and fail the test.

test/functional/mempool_expiry.py

@@ -13,6 +13,7 @@ definable expiry timeout via the '-mempoolexpiry=<n>' command line argument
 from datetime import timedelta
 
 from test_framework.blocktools import COINBASE_MATURITY
+from test_framework.messages import DEFAULT_MEMPOOL_EXPIRY_HOURS
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import (
     assert_equal,
@@ -20,7 +21,6 @@ from test_framework.util import (
 )
 from test_framework.wallet import MiniWallet
 
-DEFAULT_MEMPOOL_EXPIRY_HOURS = 336  # hours
 CUSTOM_MEMPOOL_EXPIRY = 10  # hours