Reorganize and clean up helper functions for kernel testing

[jeremykubica]

Move the sigmaGFilteredIndices which is only used to test the on-GPU implementation of sigmaG filtering into a better named file (filtering.cpp -> kernel_testing_helpers.cpp) to make it clear that this function only exists for testing purposes. This file also creates a single location where we can add more such testing functions in the future. Remove the filtering.cpp and filtering.h files as they did not end up being used outside that one testing use case. Removes the unused calculateLikelihoodFromPsiPhi function and its tests as well.