Image tests should not rely on Classy

Here's a patch. The ones that needed the class->stark change were

Functional/ImageStyles/ConvertTest
Functional/ImageDimensionsTest
Functional/ImageFieldDisplayTest

The rest of the changes are status message assertions I'm trying to refactor as I go through these classy issues. Hopefully they are not too distracting.

It's noteworthy that there is a Functional JS test with the following code:

    $this->assertSession()->waitForElement('css', '.messages--error');

    // Verify that Ajax validation messages are displayed only once.
    $this->assertSession()->elementsCount('xpath', '//div[contains(@class, "messages--error")]', 1);

First, the JsWebAssert status message methods can't count, which is what this assertion is about. Second, the assertion works because the message is added by js rather than by the theme. Probably file.js, but I'm not 100%.

Also worth calling out there's a @todo in one of the updated files that I considered handling here:

    // @todo Uncomment this once
    //   https://www.drupal.org/project/drupal/issues/2670966 is resolved.
    // $this->assertEquals('<img src="' . $url . '" width="41" height="41" alt="" class="image-style-test" />', $this->getImageTag($variables));

This will need to be modified after it's uncommented in order to remove the class. The problem is there are two more assertions a few lines down that are unrelated to classy/stark but fail when I uncomment them. So it's probably better to make the issue to act on the todo and not worry about it here.

failure in Drupal\Tests\ckeditor\FunctionalJavascript\CKEditorIntegrationTest. Resetting status.