Solvers C++ unittests (unconstrained)

[skleff1994]

Implemented C++ unittests for all solvers following the Crocoddyl testing machinery and fixed some bugs.
Specifically:

Added a KKT solver with modified termination criteria for testing purposes
Adapted the Crocoddyl's unittest factory for our testing needs
Fixed initial state bug in SolverSQP, SolverCSQP, SolverCSQP_PROXQP
Allowed varying (nx,nu) in SolverCSQP_PROXQP to avoid segfault in the LQR test case

All unittests passed in Debug & Release modes on my machine.
Run ctest -v from the build directory to run the tests on your machine.