Fix multiwallet test to look for proper jsonrpc text

Summary: N/T

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

Reviewers: deadalnix, jasonbcox, #bitcoin_abc, schancel

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1032