Merge #12079: Improve prioritisetransaction test coverage

Summary:
7f67dd0aa6 [qa] Improve prioritisetransaction functional test (João Barbosa)

Pull request description:

Tree-SHA512: 7a5c446772069cd9ace085ae2635e1f61870c597e2216614628f4b6ebfe209b29f381a182a6f60d09f43f22bb82b59bb573b5441fa8e7b958a5fd0d5aad80d86

Backport of Core PR12079
https://github.com/bitcoin/bitcoin/pull/12079/

Test Plan:

test_runner.py mining_prioritisetransaction

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5292