test: Use COINBASE_MATURITY in functional tests
Summary:
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!
Backport of core#21989.
Test Plan:
With chronik:
ninja check-functional-extended
Reviewers: #bitcoin_abc, roqqit
Reviewed By: roqqit
Subscribers: roqqit
Differential Revision: https://reviews.bitcoinabc.org/D16385