Support running tests under .Net Core and other platforms

[jwosty]

Also, other .Net framework versions.

This should be pretty easy. Steps required:

1. Create stub test runner server host projects for each platform, using Expecto.RunnerServer.Net461.fsproj as a basis, and add each as a dependency to the main addin
2. Go into the code for RemoteTestRunner (inside TestDiscoverer.fs) and add logic to the launching code to launch a different exe based on which platform the user wants to run under (either that or based off of the target framework of the user's project being built)