CustomDefaultAssembliesToScan.cs

AppDomainAssemblyTypeScanner.DefaultAssembliesToScan = new Func<Assembly, bool>[]
{
    x => x == typeof(NancyEngine).Assembly,
    x => ShouldReference(x)
};

public static bool ShouldReference(Assembly assembly)
{
    var assemblyName = assembly.GetName().Name;

    if (assemblyName.EndsWith(".mm.dll")) return false;

    return !assemblyName.StartsWith("Nancy.Testing",StringComparison.OrdinalIgnoreCase) &&
        assembly.GetReferencedAssemblies().Any(r => r.Name.StartsWith("Nancy", StringComparison.OrdinalIgnoreCase));
}