Added back image copy for triangle_auto_threshold test
Added back image copy for triangle_auto_threshold test

In Python3 the output filename for the triangle_auto_threshold test
differs from Python2. Added a logical test for Python2/3 outputs. The
test will not fail if either output name does not exist, it just won’t
move the output files.

stiphyMT/plantcv,stiphyMT/plantcv,danforthcenter/plantcv,danforthcenter/plantcv,stiphyMT/plantcv,danforthcenter/plantcv