[qa] fundrawtransaction to use billable_size instead of count_bytes

Summary:
Replace count_bytes for billable_size to properly calculate resultant_fee_rate
when billable_size actually changes from being the transaction size.

Depends on D1645

Test Plan:

./test/functional/test_runner.py fundrawtransaction

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1868