test: run mempool_spend_coinbase.py even with wallet disabled
Summary:
mempool_spend_coinbase.py is not a wallet test, so it should run even when the wallet is not compiled.
This is a backport of core#20385
Test Plan: ninja check-functional
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D10805