Move to use dotnet CLI 2.0

[dsyme]

• Use new style project files
• Test using XUnit
• Compile the TPSDK as .NET Standard 2.0 and .NET Framework 4.6.1
• Test the TPSDK using .NET Framework 4.6.1

We can't test the TPSDK using .NET CoreApp 2.0 (or host it in the F# compiler running on .NET Core) because of dotnet/fsharp#3658 (comment)

[dsyme]

Having trouble getting this through Travis, but they are not directly related to the PR, but rather to the state of using new-style .NET project files on travis.

Have contacted @enricosada and @nosami about this

[enricosada]

I'll check travis failure.

btw if you need it, nunit works already with .net core.
you can install the template (xunit is bundled) with https://github.com/dotnet/templating/wiki/Available-templates-for-dotnet-new

dotnet new -i "NUnit3.DotNetNew.Template::*"

and dotnet new nunit.

usual dotnet test works

[dsyme]

@enricosada Thanks. I've learned a sudden dislike for XUnit after I discovered it doesn't record console output.