BUG: Avoid SimpleImageRegistration{Float,Double}Test output clobbering.

Fixes test failures on parallel builds.

Change-Id: I5ab4d8acf9cf7984185e81a2c0492465d83bdebc
Merge topic 'registration-double-float'

1ae8526 BUG: Avoid SimpleImageRegistration{Float,Double}Test output clobbering.