Injecting external system with poor man's DI

PoorMansIntegrationTest.cs

[TestFixture]
public class DefaultIntegrationTests
{
    [Test]
    public void SetSomethingLabel()
    {
        const string response = "something";
        var classA = new ClassA(new ClassB(new MockRestService { Response = response }));
        var defaultPage = new _Default(classA);
        defaultPage.Page_Load(this, new EventArgs());
        Assert.That(defaultPage.SomethingLabel.Text, Is.EqualTo(response));
    }
}

public class MockRestService : IRestService
{
    public string Response { get; set; }
    public string GetDataFromRestService()
    {
        return this.Response;
    }
}