E2E tests: Fix Slack integration test failure

[mauromalara]

Description

After the debugging and testing achieved in #3274, we could see that test_slack_integration was failing, so we must find the reason for the failure and a solution for it.

The reason for failure is that the alert has not been sent to Slack and the possible solution is to wait for the regex to match the alert (the test always waits 5 seconds)

Executions

• E2E Tests: Mark unstable tests as skipped #3275

[damarisg]

We decided to close them as it was not planned.