Improve stability of mirror zone system tests

Merged Michał Kępień requested to merge michal/improve-stability-of-mirror-zone-tests into master

This MR contains a few rather nuanced tweaks for the "mirror" system test. To the best of my knowledge, the pitfalls addressed by this MR have not yet caused a test failure in GitLab CI, but are nevertheless required for reliably preventing races when waiting for log messages and for making the test's correctness independent of specific zone names.

See commit log messages for the rationale behind each change.

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