<PackageReference Include="NUnit" Version="3.8.0" />

AssemblyExtensions

public static class AssemblyExtensions
Extensions for Assembly that are not available in .NET Standard
public static object[] GetCustomAttributes(this Assembly asm, Type attributeType, bool inherit)

DNX does not have a version of GetCustomAttributes on Assembly that takes an inherit parameter since it doesn't make sense on Assemblies. This version just ignores the inherit parameter.

public static IList<Type> GetTypes(this Assembly asm)

Gets the types in a given assembly