[backport#15758] test: Add getunconfirmedbalance test with conflicts

Summary:
https://github.com/bitcoin/bitcoin/pull/15758/commits/fa195315e686f7d77e7fd5c1fd54485ccc8116f0

Depends on D6440

Partial backport of Core PR15758

Test Plan:

test_runner.py wallet_balance

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6441