Use COINBASE_MATURITY constant in functional tests.

test/functional/mempool_expiry.py

@@ -12,6 +12,7 @@ definable expiry timeout via the '-mempoolexpiry=<n>' command line argument
 
 from datetime import timedelta
 
+from test_framework.blocktools import COINBASE_MATURITY
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import (
     assert_equal,
@@ -36,7 +37,7 @@ class MempoolExpiryTest(BitcoinTestFramework):
 
         # Add enough mature utxos to the wallet so that all txs spend confirmed coins.
         self.wallet.generate(4)
-        node.generate(100)
+        node.generate(COINBASE_MATURITY)
 
         # Send a parent transaction that will expire.
         parent_txid = self.wallet.send_self_transfer(from_node=node)['txid']