Flacky CI due to Slack rate limit

We get some false negative on the tests because we test against a real Slack API:

> JoliCode\Slack\Exception\SlackErrorResponse: Slack returned error code "ratelimited"

I see two subject here:

- how can we, as a Slack API client, handle those rate limit? Watch the Retry after header?
- should it be only on the tests? or global?
- make the CI green