How can I use Xamarin.Forms in a NUnit Library project?

I wanted to test out some binding code using BindableObject in a NUnit Library project, but I keep getting the error

You MUST call Xamarin.Forms.Init(); prior to using it.

The problem is, that based on the source code in the assembly browser, that function is only defined in Xamarin.Froms.Platform.iOS, which I do not have in the NUnit project. How can I write simple unit tests with Xamarin.Forms? What kind of project should I use for that?