By default posting form data from a SilverStripe unit test seems to restrict your POST data to only valid values. Meaning, if you want to post a value for a select field on the form, the value must be in the options for that select field in order for it to be POST'd. [ad#Half Banner] Usually when submitting form data from a unit test its easiest to use the FunctionalTest->submitForm() function with something like:

$this->get(Director::makeRelative($page->Link())); 
$this->submitForm('Form_SomeForm', null, array(
  'Quantity' => 1
));

But, for sending any POST data you like to a form the easiest way I have found is to grab the existing form data in an array format and then use FunctionalTest->post() to send that data, that way you can set any values you like in the data array, e.g:

/**
 * Helper to get data from a form.
 * 
 * @param String $formID
 * @return Array
 */
function getFormData($formID) {
  $page = $this->mainSession->lastPage();
  $data = array();
  
  if ($page) {
    $form = $page->getFormById($formID);
    if (!$form) user_error("Function getFormData() failed to find the form {$formID}", E_USER_ERROR);

    foreach ($form->_widgets as $widget) {

      $fieldName = $widget->getName();
      $fieldValue = $widget->getValue();
	    
      $data[$fieldName] = $fieldValue;
    }
  }
  else user_error("Function getFormData() called when there is no form loaded.  Visit the page with the form first", E_USER_ERROR);
  
  return $data;
}

function testSomething() {

  $data = $this->getFormData('Form_SomeForm');
  $data['SomeFormField'] = 3;
  
  $this->post(
    Director::absoluteURL($page->Link()),
    $data
  );
}

Its not the best method, I'm pretty sure the _widgets variable is intended to be private lol, but it works.