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.
(That's the approach I had in c2a22b57b07968d3bce2cedd45cd3d5a556f0e86, but then I realized it was just running the net45 tests twice.)