[qa] change abandonconflict to explicitly calculate fees

Summary: As per title

Test Plan:

./test/functional/test_runner.py abandonconflict

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1552