BUG: Two tests try to write the same file

itkMorphologicalGradientImageFilterTest and
itkMorphologicalGradientImageFilterTest2 both try to write to the
same output file. On a system with multiple threads, the tests may
both try to write at the same time creating a truncated file.

Change-Id: Id9ae289ceb9216d315d6dbb809ab366b0f6eb5da
Merge topic 'DuplicateTestImage'

65b01ef2 BUG: Two tests try to write the same file