File tests should not rely on Classy

Simple patch attached.

It may be of interest to note that there is a Function JS test featuring the following lines.

$elements = $page->waitFor(10, function () use ($page) {
      return $page->findAll('css', '.messages--error');
    });
    $this->assertCount(1, $elements, 'Ajax validation messages are displayed once.');

This looked like a candidate for refactoring with the new JsWebAssert::statusMessageExistsAfterWait, but that method cannot count the number of message, which is a key aspect of this assertion.

The reason this assertion works even though that test uses stark is that this message is put on the page by file.js, not the theme. It's an ajax thing.

Looks like a random fail in a quickedit test. Resetting status.