Run tests on both k10 and net45

Third time's a charm...

@davidfowl @loudej Would you mind glancing through this? I modified the k task to take an optional runtime parameter. If specified, it basically does a k list and picks one with the specified runtime and the same version and architecture as the active one.
It's not guaranteed to be in the path. build.cmd ensures k and kpm are, but not kvm. Specifically, the CI agents, don't have it in the path.