var asm = new FakeAssembly(x => {
    x.AddResource("foo", "bar");
    x.AddResource("bar", "foo");
});

public class FakeAssembly : Assembly
{
    private readonly NameValueCollection resources = new NameValueCollection();

    public FakeAssembly()
    {
    }

    public FakeAssembly(Action<FakeAssemblyConfigurator> closure)
    {
        var configurator =
            new FakeAssemblyConfigurator(this);

        closure.Invoke(configurator);
    }

    public override string[] GetManifestResourceNames()
    {
        return this.resources.AllKeys;
    }

    public class FakeAssemblyConfigurator
    {
        private readonly FakeAssembly assembly;

        public FakeAssemblyConfigurator(FakeAssembly assembly)
        {
            this.assembly = assembly;
        }

        public FakeAssemblyConfigurator AddResource(string name, string value)
        {
            this.assembly.resources.Add(name, value);
            return this;
        }
    }
}