Unit testing Rx binding with UITextView

Question

I have a ControlProperty binding like this in my UIViewController:

textView.rx.text.orEmpty.asObservable()
    .bind(to: viewModel.descriptions)
    .disposed(by: disposeBag)

, where viewModel.descriptions is of type BehaviorRelay<String>.

In a TDD approach, I would like to write an (initially failing) test (as the initial step of the red-green-refactor cycle) to see that this binding exists. I tried with a test code like this:

sut.textView.insertText("DESC")
sut.textView.delegate?.textViewDidChange?(sut.textView)

expect(mockViewModel.lastDescriptions).to(equal("DESC"))

, where sut is my ViewController and mockViewModel contains code to subscribe to the descriptions BehaviorRelay:

class MockViewModel: MyViewModelProtocol {
    var descriptions = BehaviorRelay<String>(value: "")
    var lastDescriptions: String?
    private var disposeBag = DisposeBag()

    init() {            
        descriptions.asObservable()
            .skip(1)
            .subscribe(onNext: { [weak self] (title: String) in
                self?.lastDescriptions = title
            })
            .disposed(by: disposeBag)
    }
}

However, I cannot push the new value to the textView so that the descriptions BehaviorRelay gets it.

How can I achieve descriptions.asObservable() to fire when entering a value in the textView? If it were a UITextField then I would do:

textField.text = "DESC"
textField.sendActions((for: .editingChanged))

Thanks in advance for any help.