Convert AJAX part of \Drupal\field\Tests\Boolean\BooleanFieldTest::testBooleanField to JavascriptTestBase

Here is a patch :)

Based on @dawehner review in previous issues here is updated patch.

Already looking nice, but couple of things:

1. +++ b/core/modules/field/tests/src/FunctionalJavascript/Boolean/BooleanFieldTest.php
   @@ -0,0 +1,193 @@
   +    // Verify that boolean value is displayed.
   +    $entity = EntityTest::load($id);
   +    $display = entity_get_display($entity->getEntityTypeId(), $entity->bundle(), 'full');
   +    $content = $display->build($entity);
   +
   +    $output = \Drupal::service('renderer')->renderRoot($content);
   +    $text = '<div class="field__item">' . $on . '</div>';
   +    $regex = '/' . preg_quote($text, '/') . '/ui';
   +    $message = sprintf('The string "%s" was not found anywhere in the HTML response of the current page.', $text);
   +    $this->assert((bool) preg_match($regex, $output), $message);
   

   This is a replacement for setRawContent() which is something we only do in Kernel tests now. So, either that has to be taken out and moved to a kernel test, or we should try to find a way to do that render on a URL we can call. The latter would be better since then we have an actual functional test. Doesn't entity_test have a detail view we can render this field on?

2. +++ b/core/modules/field/tests/src/FunctionalJavascript/Boolean/BooleanFieldTest.php
   @@ -0,0 +1,193 @@
   +    $assert_session->waitForElement('css', '.field-plugin-summary-cell > .ajax-new-content');
   

   All 'waitfor' methods should be followed by an assertion that the element is actually on the page, otherwise we might just be waiting 10 seconds for nothing.
   I think all the other waits already have this, just missed this one :)

I'm surprized that whole BooleanFieldTest can be converted to BrowserTestBase :) So, no Javascript dependency.
Here is a patch.

p.s. Based on @vaplas comment here #2809493: Convert AJAX part of \Drupal\field\Tests\Boolean\BooleanFormatterSettingsTest::testBooleanFormatterSettings to JavascriptTestBase

Agree, thanks for review. Here is re-rolled patch with 8.6.x.

Re-rolled patch was failed first time because of some another unrelated issue, now it's passed. Moved again to RTBC based on #10

Committed/pushed to 8.6.x and cherry-picked to 8.5.x. Thanks!