Add `SuccessfulTests` to ensure something runs in every non-Helix run (#22406)

* Remove extra `[SkipOnHelix]` attribute
- only need the one compiled into Microsoft.AspNetCore.Testing
- update the documentation to reflect this
  - nit: address Markdown warnings that VS Code showed

* Add `SuccessfulTests` to ensure something runs in every non-Helix runs
- #22241
- cleans up hundreds of warnings but leaves a couple for the Blazor tests assembly
  - see comments about xUnit runner command line in the new class