Fix and improve BSSRDF unit tests
Fix and improve BSSRDF unit tests

glebmish/appleseed,pjessesco/appleseed,Vertexwahn/appleseed,Biart95/appleseed,est77/appleseed,Aakash1312/appleseed,Biart95/appleseed,luisbarrancos/appleseed,aytekaman/appleseed,aiivashchenko/appleseed,Vertexwahn/appleseed,Biart95/appleseed,gospodnetic/appleseed,appleseedhq/appleseed,luisbarrancos/appleseed,appleseedhq/appleseed,luisbarrancos/appleseed,Aakash1312/appleseed,gospodnetic/appleseed,aiivashchenko/appleseed,Aakash1312/appleseed,est77/appleseed,Aakash1312/appleseed,est77/appleseed,est77/appleseed,dictoon/appleseed,glebmish/appleseed,aiivashchenko/appleseed,dictoon/appleseed,dictoon/appleseed,Biart95/appleseed,aiivashchenko/appleseed,aiivashchenko/appleseed,pjessesco/appleseed,gospodnetic/appleseed,appleseedhq/appleseed,gospodnetic/appleseed,Aakash1312/appleseed,glebmish/appleseed,est77/appleseed,glebmish/appleseed,gospodnetic/appleseed,pjessesco/appleseed,luisbarrancos/appleseed,pjessesco/appleseed,appleseedhq/appleseed,pjessesco/appleseed,aytekaman/appleseed,Biart95/appleseed,dictoon/appleseed,Vertexwahn/appleseed,appleseedhq/appleseed,aytekaman/appleseed,aytekaman/appleseed,dictoon/appleseed,Vertexwahn/appleseed,Vertexwahn/appleseed,aytekaman/appleseed,luisbarrancos/appleseed,glebmish/appleseed