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>();
}