My preferred unit testing framework for .Net Core, xUnit, has had quite a few changes for the 2.3 release. One of them was how the tests libraries themselves are executed and how the framework version was determined.
This becomes a problem when you have multiple frameworks for version 2.0 installed, e.g. 2.0.0 and 2.0.3. The compiler sets the version of .Net Core App to 2.0 when you target netcoreapp2.0, but then fails to resolve it to 2.0.0. livarcocc explains it here. You'll receive error messages like this when you attempt to run xUnit:
This isn't a bug within xUnit, but rather with the dotnet CLI, but xUnit still has a patch for this planned. In the meantime, you can set the RuntimeFrameworkVersion attribute in your *.csproj file to a fixed version when running tests for netcoreapp2.0: