Close unit-test milestone

The unit test list should be as follows:

1. DDP converges in 1 step for LQR systems
2. DDP and KKT for LQR matches
3. num-diff versus algo diff
4. stopping criteria unit test (tbf, possibly compare with KKT)
5. regularization unit test (test that backward pass with regularization or with control cost is equal)
6. regularization tuning test (tbf)

We may also want to add an integrative test with the arm (and later with legged robots), i.e. comparing the numerical results of the test with a reference execution.

Unit-test validation before merge-request, implemented by @gsaurel should be active.

When all that points are matched, close the milestone unittest.