Add miner fund whitelist unit tests

Summary: Introduce a unit test file for the miner fund since we will be doing a lot of refactoring and regressions are unacceptable.

Test Plan:

ninja check-bitcoin-minerfund_tests

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D13075