Fix mining_prioritisetransaction

Summary: The core PR10196 backported in D3386 is fubared.

Test Plan:

./test/functional/test_runner.py mining_prioritisetransaction

Reviewers: #bitcoin_abc, deadalnix, markblundeberg

Reviewed By: #bitcoin_abc, markblundeberg

Subscribers: markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D3400