Add Lancelot case based on shadowCasting manual test

One important part here is that the cube receives no shadows, meaning
no shadow contribution should be visible on its top face.

Task-number: QTBUG-82957
Change-Id: Ic79291f588d4168dacbf38bec6a7790a54c04310
Reviewed-by: Andy Nichols <andy.nichols@qt.io>