Cleanup Renderer Tests

[quarkus]

Task Breakdown:

1. rename vanilla-renderer-tests -> template-element-tests
2. cover all test cases from "example element" (& more)
3. reuse Element Definitions between Tests & Test Files, maybe eben (demos / prototyping) !?
4. pay attention on readability of tests, comment maybe !?

what about vanilla-render-slots !!?