Cleanup no-contact-pair case in contactConstraints
ABCDC respects kinsol contract.

AllBodiesClosestDistanceConstraint (ABCDC) no longer re-runs `doKinematics`.
`testKinCnst_userfun_userfun` now has an additional flag that specifies whether
mex dynamics should be used throughout - even when testing a non-mex
constraint. This flag is only set to true for ABCDC right now. The behavior of
`testKinCnst` for the other constraint classes is unchanged.