New methods for Javascript testing

New methods have been added to Drupal\FunctionalJavascriptTests\JSWebAssert to facilitate testing javascript driven events that added new elements to the page. There are:

• ::waitForElement()
• ::waitForElementVisible()
• ::waitForButton()
• ::waitForLink()
• ::waitForField()
• ::waitForId()

Additionally we discourage the use of:

• ::assertWaitOnAjaxRequest()
• ::waitOnAutocomplete()

Example usage in a JavascriptTestBase test:

    $assert_session = $this->assertSession();

    // Do something that makes an element visible.
    $result = $assert_session->waitForElementVisible('named', array('button', 'Added WaitForElementVisible'));
    $this->assertNotEmpty($result);

    // Do something that adds a button.
    $result = $assert_session->waitForButton('Added button');
    $this->assertNotEmpty($result);

    // Do something that adds a link
    $result = $assert_session->waitForLink('Added link');
    $this->assertNotEmpty($result);

    // Do something that adds a field.
    $result = $assert_session->waitForField('added_field');
    $this->assertNotEmpty($result);

    // Do something that adds an HTML element with a particular ID.
    $result = $assert_session->waitForId('js_webassert_test_field_id');
    $this->assertNotEmpty($result);