public async Task CanRegisterNewUser()
        {
            var vm = await GetSut();

            NavigationAdapter.Should().BeDisplaying <AddUserViewModel>();

            vm.AddNewUserCommand.Should().BeDisabled();

            vm.EmailAddress    = "*****@*****.**";
            vm.Firstname       = "John";
            vm.Lastname        = "Doe";
            vm.Password        = "******";
            vm.ConfirmPassword = vm.Password;

            vm.AddNewUserCommand.Should().BeEnabled();

            vm.AddNewUserCommand.Click();

            vm.AddNewUserCommand.Should().BeDisabled();

            CredentialService.Should().HaveRegistered("John", "Doe");

            NavigationAdapter.Should().NotBeDisplaying <AddUserViewModel>();
        }