[qa] use calculate_fee for prioritise_transaction

Summary: As per title. Prepare this for fee calculation changes.

Test Plan:

./test/functional/test_runner.py prioritise_transaction

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix, jasonbcox

Subscribers: deadalnix, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1587