fix(tests): preventing test failure slip-througs to main

[mfranzke]

The checks for a PR run through smoothly, but it's results would lead to a fail on main branch. We need to prevent this to happen. This is caused by the optimizations that we made with PR #4452, and totally removed this condition with #4908 as a quick solution.

https://github.com/db-ux-design-system/core-web/blob/main/.github/workflows/02-e2e-showcases.yml#L77-L85