[Chronik] Add a merge lock to avoid a race in the tests
Summary:
Avoid a race when several tests are testing merge errors, similar to what's done outside of tests.
Extracted from D14817 by @tobias_ruck.
Test Plan:
./contrib/teamcity/build-configurations.py build-chronik
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Subscribers: tobias_ruck
Differential Revision: https://reviews.bitcoinabc.org/D14983