Handle fsx file with dotnet core 3

[rfrerebe]

#1886
Added a new test project for Fable.Cli

[rfrerebe]

Hello @alfonsogarciacaro ,

You can merge this PR if you wish. New tests are being run by factory.
Ideally, it would be cool to also add to test fsx build, but that would need adding paket in your build process.
Other option is to wait for fsx files to handle references directly (see this PR dotnet/fsharp#5850)

Let me know, if you are interested

[alfonsogarciacaro]

Thanks @rfrerebe!

It's good to have some kind of integration tests, I've been too lazy for that :) If you want to test the compilation of an .fsx script using Paket, you should only need to include Paket in the directory of the test project without touching the overall repo structure, right? If that's the case, I'm OK with you adding Paket in a folder within the tests directory 👍