Measure flaky tests

[yhatt]

Marp CLI is testing actual behavior of conversion through browsers (and LibreOffice). But tests on CI often fail by timeout error by tests that are doing browser conversion. This issue is to measure, track, and deal with frequent flaky tests.

ToDo

• Set up Codecov test analytics
  • https://about.codecov.io/blog/find-failing-and-flaky-tests-with-codecov-test-analytics/