Add custom flaky decorator to handle unstable tests

Add custom flaky decorator to handle unstable tests

If the flaky plugin for pytest is available, use its decorator to
support re-running unstable tests. In case the package is missing,
execute the test as usual without attempts to re-run it in case of
failure.

This is mostly intended to increase the test stability in CI. Using a
custom decorator enables us to keep the flaky package as an optional
dependency.

(cherry picked from commit 5b703de733b1f53f0452e8e654459d7adeb9aa00)