TestOrchestrator.cs

var contextMock = new Mock<DurableOrchestrationContextBase>();
var logMock = new Mock<ILogger>();
var input = new MyClass();
var result = new MyOtherClass();

contextMock.Setup(context => context.GetInput<EmailInput>()).Returns(input);
contextMock.Setup(context => context.CallActivityAsync<EmailResult>("A_Send", input)).Returns(Task.FromResult<EmailResult>(result));

var orchestrationResult = await Orchestrators.EmailSender(this.contextMock.Object, this.logMock.Object);