[qa] allow assert_fee to have a bit more wiggleroom

Summary: As per title

Test Plan:

make check && ./test/functional/test_runner.py

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix, jasonbcox

Subscribers: jasonbcox, deadalnix, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1858