simple_string_submit example

  public function testExecuteSubmitHandlers() {
    $form_submitter = $this->getFormSubmitter();
    $mock = $this->prophesize(MockFormBase::class);
    $mock
      ->hash_submit(Argument::type('array'), Argument::type(FormStateInterface::class))
      ->shouldBeCalledOnce();
    $mock
      ->submit_handler(Argument::type('array'), Argument::type(FormStateInterface::class))
      ->shouldBeCalledOnce();
    $mock
      ->simple_string_submit(Argument::type('array'), Argument::type(FormStateInterface::class))
      ->shouldBeCalledOnce();

    $form = [];
    $form_state = new FormState();
    $form_submitter->executeSubmitHandlers($form, $form_state);

    $form['#submit'][] = [$mock->reveal(), 'hash_submit'];
    $form_submitter->executeSubmitHandlers($form, $form_state);

    // $form_state submit handlers will supersede $form handlers.     $form_state->setSubmitHandlers([[$mock->reveal(), 'submit_handler']]);