Add helper to wait for validation interface queue to catch up

Summary:
Partial backport of core PR11824 (commit 97d2b09)

Depends on D2610
Completes T544

Test Plan:

make check
./test/functional/test_runner.py --extended

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2611