fix fee estimation bug in functional tests

Summary:
D1926 introduced this magic number of 81 which doesn't seem to have any
justification (it's definitely larger than any signature can possibly be),
It looks like it was a rushed fix to the CI.

This number was actually too small, and dbcrash only ended up working
because of the fLimitFree being disabled for rpc-submitted transactions.

Test Plan:

test_runner.py --extended

Reviewers: jasonbcox, #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4729