[qa] use billable_size is testnode fee estimates
[qa] use billable_size is testnode fee estimates

Summary: As per title. Depends on D1858.

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

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1859

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