Improve test coverage with practical data sets

[tbirdso]

Several ultrasound image filters rely on contrived data for CTest coverage. For example, Spectra1DAveragingImageFilter tests currently average over multiple copies of a single image, spectra_pixel_image.mha, which depicts a 3D image with a single 31-channel pixel. In this case limited test data failed to cover an issue with stepping over lines, leading to the patch in #187.

It would be useful to revisit our testing and potentially duplicate some image filter tests with a small variety of input data samples representing real-world cases.