BUG: No DiscreteHessianGaussianImageFunctionTest.

The itkDiscreteHessianGaussianImageFunctionTest is restored in this patch.

Checks have been added to bring the code coverage to 100%.

Error checking for images with zero spacing has been removed, since the
ImageBase class alreay performs those checks, making impossible for the error
to happen, and also making impossible to create a test case for the error case
previously considered in the original test.

Make use of the TestingMacros, so simplify the code.

Change-Id: I013cb7faf171880b1e4bb6b8cad1531e1af580ef
Merge topic 'itkDiscreteHessianGaussianImageFunction_CodeCoverage'

465355f5 BUG: No DiscreteHessianGaussianImageFunctionTest.