Using a fake implementation 
public class FakeResult<T> : IResult<T> {
    public bool Success {
        get { return true; }
    }
}
along with adding a TypeRelay customization 
 fixture.Customizations.Add(new TypeRelay(typeof(IResult<>), typeof(FakeResult<>)));
All calls for IResult<> will use the FakeResult<> which has its Success to return trueno matter what is the type of T.
Full example to test that the mock works as intended.
[TestClass]
public class AutoFixtureDefaultGeneric {
    [TestMethod]
    public void AutoFixture_Should_Create_Generic_With_Default() {
        // Arrange
        Fixture fixture = new Fixture();
        fixture.Customizations.Add(new TypeRelay(typeof(IResult<>), typeof(FakeResult<>)));
        //Act
        var result = fixture.Create<IResult<string>>();
        //Assert
        result.Success.Should().BeTrue();
    }
}