Ensure missing test warning fails on CI.

Summary: Depends on D1075

Test Plan: TEAMCITY_VERSION=1 ./test/functional/test_runner.py

Reviewers: deadalnix, jasonbcox, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1076