PyTest for SingleFreqFFT

[ToddSchumannNTIA]

The code could use a PyTest for the individual actions, especially those that apply some form of processing (namely single frequency FFTs). We should generate a signal IQ, feed that into the processing algorithm and ensure the returned result is what we expect. This would prevent any accidental mistakes when adjusting the algorithms. Once the Generic Measurement Info branch is pulled in, this can be implemented (this issue is serving as a reminder).