C# Reflection to find classes that implement interface with multiple open generic parameters

Let's say I have an interface like this:

public interface ITest<T1, T2>
{
}

And a class like this:

public class Concrete : ITest<int, string>
{
}

Using reflection, how can I find this concrete class? I've tried the following but it doesn't work because I'm not specifying the generic type parameters.

var concretes = Assembly.GetAssembly(typeof(Concrete)).GetTypes()
                        .Where(x => x.IsAssignableFrom(typeof(ITest<,>))
                               && !x.IsInterface);

This returns zero items. Is it even possible to do what I'm trying to do?