[qa] make fundrawtransaction test calculate fees properly

Summary: As per title

Test Plan: ./test/functional/test_runner.py fundrawtransaction

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1635