How do you unit test an interface?

If you want to run the same tests against different implementers of your interface using NUnit as an example:

public interface IMyInterface {}
class A : IMyInterface { }
class B : IMyInterface { }
class C : IMyInterface { }

public abstract class BaseTest
{
    protected abstract IMyInterface CreateInstance();

    [Test]
    public void Test1()
    {
        IMyInterface instance = CreateInstance();
        //Do some testing on the instance...
    }

    //And some more tests.
}

[TestFixture]
public class ClassATests : BaseTest
{
    protected override IMyInterface CreateInstance()
    {
        return new A();
    }

    [Test]
    public void TestCaseJustForA()
    {
        IMyInterface instance = CreateInstance();   
        //Do some testing on the instance...
    }

}

[TestFixture]
public class ClassBTests : BaseTest
{
    protected override IMyInterface CreateInstance()
    {
        return new B();
    }
}

[TestFixture]
public class ClassCTests : BaseTest
{
    protected override IMyInterface CreateInstance()
    {
        return new C();
    }
}