qa: Improve getchaintxstats functional test

Summary:
Completes T561.
Depends on D2711.
Backport of PR12083
https://github.com/bitcoin/bitcoin/pull/12083/files

Test Plan:
make check
test_runner.py

Reviewers: Fabien, deadalnix, O1 Bitcoin ABC, #bitcoin_abc, nakihito

Reviewed By: Fabien, O1 Bitcoin ABC, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2702