Merge bitcoin/bitcoin#21989: test: Use COINBASE_MATURITY in functional tests

bfa9309ad6 Use COINBASE_MATURITY constant in functional tests. (Kiminuo)
525448df9d Move COINBASE_MATURITY from `feature_nulldummy` test to `blocktools`. (Kiminuo)

Pull request description:

  `COINBASE_MATURITY` constant was added to `feature_nulldummy` test in #21373. This PR moves the constant to `blocktools.py` file and uses the constant in more tests as suggested [here](https://github.com/bitcoin/bitcoin/pull/21373#discussion_r605418462).

  Edit: Goal of this PR is to replace integer constants with `COINBASE_MATURITY` but not necessarily in *all* cases because that would mean to read and fully understand all tests. That's out of my time constraints. Any reports where `COINBASE_MATURITY` should be used are welcome though!

ACKs for top commit:
  theStack:
    ACK bfa9309ad6 🌇

Tree-SHA512: 01f04645f05a39028681f355cf3d42dd63ea3303f76d93c430e0fdce441934358a2d847a54e6068d61932f1b75e1d406f51859b057b3e4b569f7083915cb317f

test/functional/rpc_net.py

@@ -11,6 +11,7 @@ from decimal import Decimal
 from itertools import product
 import time
 
+from test_framework.blocktools import COINBASE_MATURITY
 from test_framework.p2p import P2PInterface
 import test_framework.messages
 from test_framework.messages import (
@@ -53,7 +54,7 @@ class NetTest(BitcoinTestFramework):
         self.wallet = MiniWallet(self.nodes[0])
         self.wallet.generate(1)
         # Get out of IBD for the minfeefilter and getpeerinfo tests.
-        self.nodes[0].generate(101)
+        self.nodes[0].generate(COINBASE_MATURITY + 1)
 
         # By default, the test framework sets up an addnode connection from
         # node 1 --> node0. By connecting node0 --> node 1, we're left with