[qa] Improve wallet-accounts test

Summary:
Add comments and

• Verify sending to a account causes getaccountaddress to generate new addresses.
• Verify sending to a account causes getreceivedbyaccount to return amount received.
• Verify ways setaccount updates the accounts of existing addresses.

Backport of Core PR11552

Test Plan:

./test/functional/test_runner.py wallet-accounts.py

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix, jasonbcox

Subscribers: jasonbcox, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D2001