[python] Add unit tests for Python native code

[duburcqa]

This is especially critical for 'dynamics' helpers because some methods are quite complex and it would be nice to validate them. It is pretty "easy" to do it. For instance, it would be possible to run first a simulation, extracting only the motor position/velocity/acceleration, and try the recover missing data.

[duburcqa]

Part of the code is now partially tested though gym_jiminy unit tests, but it is clearly not enough, considering that nearly 40% of the whole code base is pure Python.