Add unit tests for convergencce

[rth]

It would be useful to add tests that would run the optimization algorithm and check that the expected convergence is reached.

For instance, maybe translating scipy/optimize/test/test_optimize.py could be a start.

I will look into it.

Edit: actually it looks like there is a number of classical functions implemented in https://github.com/argmin-rs/argmin-testfunctions already..