Write unit tests for birdsong feature extraction

[PaulEcoffet]

birdsonganalysis lacks unit tests to assert that the song feature it computes are correct.
The tests should be extensive to show that it works whatever the fft_step, fft_size and freq_range chosen.

The results should be tested against signal built to match a certain value of amplitude, entropy… and maybe against results from a viable implementation on real bird songs.