Add tests for comments CRUD for REST

Attached patch is an old attempt. Maybe a good starting point. Let's see what breaks.

1. +++ b/core/modules/rest/src/Tests/CreateTest.php
   @@ -224,6 +235,53 @@ public function testCreateNode() {
   +      $entity_values['entity_id'] = $host_id;
   

   Does the host (resttest) content type supports comments?

2. +++ b/core/modules/rest/src/Tests/RESTTestBase.php
   @@ -210,6 +210,14 @@ protected function entityValues($entity_type) {
   +          'entity_type' => 'node',
   

   This misses at least

    'field_name' => 'comment',
   

Checking with \Drupal\comment\Tests\CommentItemTest it adds a comment field.

    $this->addDefaultCommentField('entity_test', 'entity_test', 'comment');