BUG: itkSingleLevelSetsv4WhitakerImage2DTest output clobber.

Prevent the two tests from writing to the same output file and possibly
clobbering each other.

Change-Id: I721f730ed6663bdf334e4ec674606509379b929a
Merge topic 'itkSingleLevelSetsv4WhitakerImage2DTest'

372dc26 BUG: itkSingleLevelSetsv4WhitakerImage2DTest output clobber.