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.
See [line 23 of build.cmd](https://github.com/aspnet/Universe/blob/dev/build-template/build.cmd#L23). I am referencing it in the same way here.