TESTS: add `is_flaky` decorator on flaky tests

[younesbelkada]

What does this PR do?

This PR introduces a new pytest decorator is_flaky that runs tests that have been decorated as being flaky. The decorator will run the test max_attempts times and if the test failed during all these attempts the test will be marked as failed.

Plan forward: identify which tests are flaky and decorate them

cc @Titus-von-Koeller

[github-actions[bot]]

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[akx]

You could also use https://pypi.org/project/pytest-retry/...

[Titus-von-Koeller]

Closing this in favor of pytest-retry approach.