how can i test my Grain which depends on another Grain?

class GrainB : Grain, IGrainB
{
    public async override Task OnActivateAsync()
    {
        var grain = GrainFactory.GetGrain<IGrainA>(0);
        await grain.Register(this.GetPrimaryKeyString());
        await base.OnActivateAsync();
    }

}

class GrainA : Grain, IGrainA
{
    public Task Register(string grainKey);
    public Task UnRegister(string grainKey);
}

there is two Grain named GrainA and GrainB. GrainB depends on GrainA. I saw the Orleans's sample and the TestGrainFactory can create a local grain which I can test. when Grain is simple, it can be tested easily. but in my problem, GrainB depends on GrainA, how can I complete the test?