disable samples position jittering when pixel decorrelation is enabled and the number of samples per pixel is 1.
cleaned up test suite: alembic.
tweaked test scenes.
cleaned up test suite: alpha.
cleaned up test suite: assemblies.
cleaned up test suite: autofocus.
cleaned up test suite: back faces.
cleaned up test suite: bokeh.
cleaned up test suite: ashikhminbrdf.
cleaned up test suite: bsdfmix.