[QA] Fix typo in fundrawtransaction test

Summary:
Backport of Core PR10069
https://github.com/bitcoin/bitcoin/pull/10069/files
Needed to fix D2517

Test Plan: test_runner.py rpc_fundrawtransaction

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2522