qa: disablewallet: Check that wallet is really disabled

Summary:
Backport core's PR10361

Depends on D1154

Test Plan:

./test/functional/test_runner.py disablewallet

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1155