[qa] wallet.py use billable_size

Summary: Update wallet.py to use billable_size prior to fee calculation changes.

Test Plan:

./test/functional/test_runner.py wallet

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1798