Additional feature/E2E testing for Auto-Merge
Context
Testing gap pointed out by @tkuah in https://gitlab.com/gitlab-sirt/shared-incidents/incident_5529/-/work_items/14#note_1943827492
We should have feature tests, or E2E tests for these scenarios
⚙ If pipeline must succeed setting is disabled if the MR auto_merge_strategy is MWCP we check for the head pipeline status ⭐ If the MR auto_merge_strategy is not MWCP, the MR is mergeable ✅ ⚙ If pipeline must succeed setting is enabled, check head pipeline status ⭐ ⭐ If the head pipeline exists If head pipeline is skipped and it's allowed state, MR is mergeable ✅ If head pipeline is success, MR is mergeable ✅ Otherwise, MR is blocked ❌ ⭐ If head pipeline does not exist, MR is blocked ❌
Proposal
Add testing coverage for the above scenarios.