I am wondering what the reason is for the kinematic coupling matrix to not be included more explicitly in unit tests for mobilizers? In particular,perhaps something in between MapVelocityToQDotAndBack and KinematicMapping tests. How do the unit tests ensure that the equation q̇ = N(q)⋅v is used when using MapVelocityToQDot?
That's a good point. We test that MapVelocityToQdot() and MapQdotToVelocity() are consistent, and we check that the N matrix is right, but we don't verify that the Map...() functions are consistent with the N matrix. That would be a good addition. If you want to file an issue describing the problem, please do. Even better if you want to submit a PR with the missing tests, that would be most welcome!