test_master_coordinator.py::test_master_selection is flaky

[juha-aiven]

What happened?

Tests failing when running Test Suite (Python 3.7) for https://github.com/aiven/karapace/pull/216. Details in the attached file logs_2793.zip.

What did you expect to happen?

Tests pass, no code changes in the PR.

What else do we need to know?

Include your platform, version, and any other information that seems relevant.