consolidate common test setup code

Rendering/ANARI/Testing/Cxx/CMakeLists.txt

Rendering/ANARI/Testing/Cxx/TestAnariAmbient.cxx

Rendering/ANARI/Testing/Cxx/TestAnariCameraInside.cxx

...g/ANARI/Testing/Cxx/TestAnariCameraInsideSmallSpacing.cxx

Rendering/ANARI/Testing/Cxx/TestAnariCellData.cxx

...ng/ANARI/Testing/Cxx/TestAnariCompositePolyDataMapper.cxx

Rendering/ANARI/Testing/Cxx/TestAnariCylinder.cxx

Rendering/ANARI/Testing/Cxx/TestAnariDynamicObject.cxx

Rendering/ANARI/Testing/Cxx/TestAnariImplicits.cxx

Rendering/ANARI/Testing/Cxx/TestAnariLayers.cxx

Rendering/ANARI/Testing/Cxx/TestAnariLights.cxx

Rendering/ANARI/Testing/Cxx/TestAnariMaterials.cxx

Rendering/ANARI/Testing/Cxx/TestAnariMultiBlock.cxx

.../Testing/Cxx/TestAnariMultiBlockPartialArrayFieldData.cxx

Rendering/ANARI/Testing/Cxx/TestAnariOrthographic.cxx

Rendering/ANARI/Testing/Cxx/TestAnariPBRMapping.cxx

Rendering/ANARI/Testing/Cxx/TestAnariPBRMaterials.cxx

Rendering/ANARI/Testing/Cxx/TestAnariPBRMaterialsCoat.cxx

Rendering/ANARI/Testing/Cxx/TestAnariPass.cxx

Rendering/ANARI/Testing/Cxx/TestAnariPassVisibility.cxx