test: Extend wallet_dump test to cover comments

Summary:
This prevent regressions n comments such as the date of the dump no longer being human readable (https://github.com/bitcoin/bitcoin/pull/17954#discussion_r406891999)

This is a backport of Core PR18597

Test Plan: test/functional/test_runner.py wallet_dump

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8923