[backport#15758] test: Add wallet_balance test for watchonly

Summary:
https://github.com/bitcoin/bitcoin/pull/15758/commits/fa464e8211850eed1b31bf1723c9e02845dacba1

Partial backport of Core PR15758

Test Plan:

./test/functional/test_runner.py wallet_balance

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6440