False positives given when calling drupalPostForm

Problem/Motivation

I am getting a 500 internal server error response but a green light in my tests when running MessageuiPermission:testMessageUiPermissions =>

php core/scripts/run-tests.sh --browser --url "http://test.drupal-8.0.3.ie" --class "Drupal\message_ui\Tests\MessageUiPermissions"

D8 version of Message UI module found at: https://github.com/mccrodp/message_ui/tree/8.x-1.x

/**
   * {@inheritdoc}
   */
  function setUp() {
    parent::setUp();

    $this->account = $this->drupalCreateUser();

    // Load 'authenticated' user role.
    $this->rid = Role::load(RoleInterface::AUTHENTICATED_ID)->id();

    // Create Message type foo.
    $this->createMessageType('foo', 'Dummy test', 'Example text.', array('Dummy message'));
  }

  /**
   * Test message_access use case.
   */
  function testMessageUiPermissions() {
    $this->drupalLogin($this->account); // User login.
    $create_url = 'admin/content/messages/create/foo'; // Set our create url.

    // Verify the user can't create the message.
    $this->drupalGet($create_url);
    $this->assertResponse(403, t("The user can't create a message."));

    // Grant and check create permissions for a message.
    $this->grantMessageUiPermission('create');
    $this->drupalGet($create_url);

    // If we get a valid response, create a message.
    if ($this->assertResponse(200, t("The user can create a message."))) {
      // Create a message at current page / url.
      $this->drupalPostForm($create_url, array(), t('Save'));
    }
}

When drupalPostForm is called I would either expect to be able to assert to a particular response 200, 404, etc., or to only get a green light on a 200, 201, 202 or successful response code. As you can see in the attached screengrab (green-light-500-error.png) I get a green light on a 500 error (drupalPostForm-error.png).

Proposed resolution

For the case this is working as designed:
Unless I am missing something, possible improvement to documentation of drupalPostForm at https://api.drupal.org/api/drupal/core%21modules%21simpletest%21src%21We...

If it is erroneous, contribute code to help fix the issue.

I am unsure why / how the code in drupalPostForm calculates $submit_matches before getting the http result code from curlExec method. i.e. in my test $submit_matches = TRUE, then curlExec gets an error 500 code. Then the following code executes and gives a green light:

// We have not found a form which contained all fields of $edit.
      foreach ($edit as $name => $value) {
        $this->fail(SafeMarkup::format('Failed to set field @name to @value', array('@name' => $name, '@value' => $value)));
      }
      if (!$ajax && isset($submit)) {
        $this->assertTrue($submit_matches, format_string('Found the @submit button', array('@submit' => $submit)));
      }
      $this->fail(format_string('Found the requested form fields at @path', array('@path' => ($path instanceof Url) ? $path->toString() : $path)));

Remaining tasks

As above

User interface changes

N/A

API changes

N/A

Data model changes

N/A