Add miner fund regression test around Wellington activation

Summary:
With the miner fund planned to be moved from consensus to policy we can start
by adding regression tests. This patch ensures the miner fund addresses
returned by getblocktemplate will be undisturbed by the transition.

Test Plan:

ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12973