E2E test with EventMesh Backend

[k15r]

Description

So far, the Eventing Manager E2E tests only cover the NATS backend szenario. To ensure full test coverage, an E2E test with EventMesh backend is needed.

Helpful Links

• GithubAction E2E test with NATS backend exists here
• E2E make target for EventMesh Backend exists and is documented here
• Reusable action is done in BTP Manager here and is called here to be executed on PRs
• The secrets for EM, IAS and Gardener need to be stored securely. This is solved in issue CI pipeline for backend switching tests #131

Acceptance

• GithubAction executes E2E test with EventMesh backend on PRs on main and release-* branches
• Action is reusable in other actions, f.ex the release flow

[grischperl]

This is not needed as the test case is covered in the backend switching test #131