dotnet test template for f# shouldnt add Program.fs for net tfm

[enricosada]

The test templates (xunit, mstest) of F# always include a Program.fs, and is needed atm ( ref dotnet/netcorecli-fsc#79 ) for netcoreapp2.0

But that's annoying swithcing to net462 because there the project is a library (by design), and the entrypoint is not needed.

Changes required: add the Program.fs under a Condition of the tfm or runtime (or better another test sdk property, if exists). like

<Compile Include="Program.fs" Condition=" '$(DontRemeberTheProp)' == '.NETCore' " />

/cc @dsyme @cartermp @KevinRansom

[dsyme]

+1, thanks

[mlorbetske]

Looks like this should be fixed with dotnet/test-templates#22