test: listsinceblock block height checks
This PR extends a listsinceblock test to check the new transaction 'blockheight' field recently added in PR17437. It also cleans up code in the test function without changing or removing existing checks.
This is a backport of Core PR18420
Test Plan: ninja && test/functional/test_runner.py wallet_listsinceblock
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D8895