[backport#15652] qa: Check unconfirmed balance after loadwallet

Summary:
https://github.com/bitcoin/bitcoin/pull/15652/commits/4bf1b1cefa9723bf2cfa8b1a938757abc99bb17b

Depends on D6247

Concludes backport of Core PR15652

Test Plan:

./test/functional/test_runner.py wallet_balance

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6248