Generate proper test WAV files

Originally, I wanted to avoid creating the test files with the system under test, this just didn't seem right.
Therefore, I created the test WAV files with `scipy.io.wavfile.write()`.

Recently, I found out that the 32-bit floating point WAV file is malformed.
See https://github.com/erikd/libsndfile/issues/70 and https://github.com/scipy/scipy/issues/3778.

I tried to find an alternative way to generate a floating point WAV file, but it's surprisingly hard to find something that doesn't involve libsndfile.

Sox produces correct files, but I didn't find a way to write specific floating-point values (like 1.0 and 0.75) to a file.

Is there a better way to create our test WAV files?

If not, I guess we'll have to create them with PySoundFile.

Another possibility would be to use only PCM test files (which can be generated correctly with many tools), but then we would have to add `dtype='float16'` in many places in the test code, which I originally wanted to avoid.