Issue when trying to create service from ContentEntity subclass and inject into another class

Question

I have a class ApplicationForm that extends ContentEntity. In my other class ApplicationFormForm (Poorly named, I know) I extend ContentEntityForm which in turn inherits from EntityForm. In one of those parent classes, a property called $entity is created, which is available in my class ApplicationFormForm.

The $entity property has all the methods I created on it and I use it in my submitForm method:

public function submitForm($form, FormStateInterface $form_state) {
 // Code omitted for brevity
 $stepsCompleted = $this->entity->getStepsCompleted();
}

Now, the problem arises when I try and unit test my submitForm method, because entity is null and therefore getStepsCompleted does not exist.

To get around this, I created an interface of my ApplicationForm class and turned ApplicationForm into a service, and injected it into my ApplicationFormForm class (type hinting the parameter as the newly created interface so I can mock it later).

The problem is, as mentioned previously, ApplicationForm inherits from other classes, and these classes have other parameters in their constructors which are not services. Specifically:

public function __construct(array $values, $entity_type) {

}

So when I try and inject my new service, it complains that I am missing argument 1 and 2 of the constructor. So... I'm pretty stuck. Does anyone have a solution so I can unit test my method?

Update - Here is my unit test. It's extending FormTestBase.

/**
   * Set up method which is called before any tests are
   * run. Mocks of the services that the class being tested rely on are
   * created. These mocks are then passed to the container, so they are
   * available to the methods under test.
   *
   * @return void
   */
  public function setUp() {

    parent::setUp();
    $container = new ContainerBuilder();

    $mockEntityTypeManager = $this
      ->getMockBuilder('\Drupal\Core\Entity\EntityTypeManager')
      ->disableOriginalConstructor()
      ->getMock();
    $container->set('entity_type.manager', $mockEntityTypeManager);

    $applicationForm = new ApplicationFormForm(
      $mockEntityTypeManager,
    );

    $this->applicationForm = $applicationForm;

    \Drupal::setContainer($container);

  }

  /**
   * @dataProvider submitFormDataProvider
   * @param $form array
   * @covers ::submitForm
   */
  public function testSubmitForm($form) {

    $form_state = new FormState();
    $form_state = $this->applicationForm->submitForm($form, $form_state);
   // Assertion will go here.

  }

  /**
   * @return array
   */
  public function submitFormDataProvider() {

    $form['base'] = array(
      '#type' => 'fieldset',
      '#tree' => true,
    );
    $form['base']['value'] = array(
      '#type' => 'textfield',
      '#title' => '<span class="form-required">Foo</span>',
      '#value' => '',
      '#attributes' => array(
        'data-required' => 'required'
      ),
      '#parents' => array(
        'base'
      )
    );

    return array(
      array($form)
    );

  }

As mentioned before, I can't see a way to mock the entity property like I have with EntityTypeManager as $entity is inherited from the parent class and has dependencies in the constructor.