Update FSharpx.Core.Tests.fsproj to build on Mac and on Windows with msbuild

Building on OS X I get the following error:

        /Users/Patrick/Projects/FSharp/fsharpx/FSharpx.Core.Tests/FSharpx.Core.Tests.fsproj: error : 
    /Users/Patrick/Projects/FSharp/fsharpx/FSharpx.Core.Tests/FSharpx.Core.Tests.fsproj: 
    /Users/Patrick/Projects/FSharp/fsharpx/FSharpx.Core.Tests/FSharpx.Core.Tests.fsproj could not import 
    "..\lib\FSharp\Microsoft.FSharp.Targets"

Also building on Windows with MSBuild I get the same error:

"C:\Projects\fsharpx\FSharpx.sln" (default target) (1) ->
"C:\Projects\fsharpx\FSharpx.Core.Tests\FSharpx.Core.Tests.fsproj" (default target) (14) ->
  C:\Projects\fsharpx\FSharpx.Core.Tests\FSharpx.Core.Tests.fsproj(37,3): error MSB4019: The imported project "C:\Projects\fsharp x\lib\FSharp\Microsoft.FSharp.Targets" was not found. Confirm that the path in the <Import> declaration is correct, and that the
file exists on disk.