Unit testing code targeting several frameworks at once with net core and xunit

I have some libraries on .net core which target "net45" and "netstandard1.6" and I want to unit-test them so I did dotnet new -t xunittest and that created a new test project targeting "netcoreapp1.0" so that it will tests only the net core code.

I tried to also compile it to target also "net45" but then I got a series of errors on test discovering. My questions is

Is there a way to test the code targeting both (maybe more later) frameworks with a single test project or should I make a test project for each one I target?