Convert PreviewTest WebTestBase to BrowserTestBase and JavascriptTestBase

Worked on this in a duplicate issue, lets move it here.

To quote myself for the other issue:
It turns out that ajax in preview doesn't actually work currently! It just shows the commands page when using any ajax link in preview. I can reproduce this in 8.7.x.
Quick test on a 8.5.3 site worked without issue so this might be a recent break.

Works fine when I do a fresh install using Unami, so my dev env was just broken

Ok so it helps if you are on the view edit page and not on the preview page when working with the preview with ajax....and the test views actually 'use ajax'.

This is an attempt at a minimal conversion, hence all the helper methods, which we can probably do without.

ok so these views are used elsewhere, lets add some new ones.

Great work! Conversion looks good.

1. +++ b/core/modules/views_ui/tests/src/FunctionalJavascript/PreviewTest.php
   @@ -0,0 +1,306 @@
   +
   +
   

   nit two blank lines here

2. +++ b/core/modules/views_ui/tests/src/FunctionalJavascript/PreviewTest.php
   @@ -0,0 +1,306 @@
   +  /**
   +   * Asserts that an element has a given class.
   +   *
   +   * @param \Behat\Mink\Element\NodeElement $element
   +   *   The element to test.
   +   * @param string $class
   +   *   The class to assert.
   +   * @param string $message
   +   *   (optional) A verbose message to output.
   +   */
   +  protected function assertClass(NodeElement $element, $class, $message = NULL) {
   +    if (!isset($message)) {
   +      $message = "Class .$class found.";
   +    }
   +    $this->assertTrue(strpos($element->getAttribute('class'), $class) !== FALSE, $message);
   +  }
   

   is this worth making a trait or in the base assert? it feels like a useful feature

Thanks for the reviews!

#16.1 fixed
#16.2 nah I don't think so, I think the usual case is that a class is used to find the element you are looking for, so you are indirectly testing for it. Directly testing for the existence of a class is not very common I think. Not often where this is a functional requirement. Also, I think, we should keep the API to a minimum when we can, less to maintain.

Let's see what Lee thinks about #17.2.

#17.2 - this could also be used to add testcoverage for #3013452: no_striping option for table rows is not respected, but only 1 other usecase not be sufficient to move this to a base class.

Committed and pushed 97ac5b70c2 to 8.7.x and 5927122ab5 to 8.6.x. Thanks!

diff --git a/core/modules/views_ui/tests/src/Functional/PreviewTest.php b/core/modules/views_ui/tests/src/Functional/PreviewTest.php
index bca8b49185..b76e464c8c 100644
--- a/core/modules/views_ui/tests/src/Functional/PreviewTest.php
+++ b/core/modules/views_ui/tests/src/Functional/PreviewTest.php
@@ -2,9 +2,6 @@
 
 namespace Drupal\Tests\views_ui\Functional;
 
-use Drupal\Component\Serialization\Json;
-use Drupal\Core\EventSubscriber\MainContentViewSubscriber;
-
 /**
  * Tests the UI preview functionality.
  *

Unused use. Fixed on commit.