EntityResource: Provide comprehensive test coverage for ContactForm entity

@benelori: thanks for taking this on! :)

Thanks @naveenvalecha!

Here's the needed test coverage.

Turns out that \Drupal\Tests\rest\Functional\EntityResource\EntityResourceTestBase::getEntityResourceUrl() is inadequate, and this has gone unnoticed for a long time.

Turns out ContactForm is the only @ConfigEntityType in Drupal core that specifies a canonical link template. This is why we're only finding this now!

That method does this:

  protected function getEntityResourceUrl() {
    $has_canonical_url = $this->entity->hasLinkTemplate('canonical');
    return $has_canonical_url ? $this->entity->toUrl() : Url::fromUri('base:entity/' . static::$entityTypeId . '/' . $this->entity->id());
  }

Simple enough, right? But … ConfigEntityBase overrides the toUrl() method:

  public function toUrl($rel = 'edit-form', array $options = []) {
    // Unless language was already provided, avoid setting an explicit language.
    $options += ['language' => NULL];
    return parent::toUrl($rel, $options);
  }

Which means that we were getting the edit-form URL instead of the canonical URL, even when we know a canonical URL exists. Fortunately, this is easy to fix.

The above patch almost passes — the only thing that's missing is the expected helpful messages for 403 responses. Fortunately, this too is trivial fix 😀

Added HAL+JSON test coverage.

Now ready for final review.

1. +++ b/core/modules/rest/tests/src/Functional/EntityResource/ContactForm/ContactFormResourceTestBase.php
   @@ -0,0 +1,97 @@
   +      'id' => 'llama',
   +      'label' => 'Llama',
   +      'message' => 'Let us know what you think about llamas',
   +      'reply' => 'Llamas are indeed awesome!',
   ...
   +      'recipients' => [],
   

   I'd like to see some coverage around the recipients field, particularly for the multi-value instance (comma separated). But that can be in the existing follow-up. It's a bit of a snowflake in so far as we're cramming multiple values into a single field.

2. +++ b/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
   @@ -1291,7 +1291,7 @@ protected function assertNormalizationEdgeCases($method, Url $url, array $reques
   -    return $has_canonical_url ? $this->entity->toUrl() : Url::fromUri('base:entity/' . static::$entityTypeId . '/' . $this->entity->id());
   +    return $has_canonical_url ? $this->entity->toUrl('canonical') : Url::fromUri('base:entity/' . static::$entityTypeId . '/' . $this->entity->id());
   

   I think a comment here to explain the findings in #13 would help - and probably prevent someone thinking that the 'canonical' argument was superfluous. I thought it was at first. Knocking back to needs work for that (docs gate), but will keep an eye out for when this gets back to RTBC so it doesn't sink to bottom of list.

1. I tried to address it here (there is no "existing follow-up" that I know of?). The comma-separated thing is only for the @ContactForm entity form. When you save that form, it stores them as a list of strings. (I tested manually to verify.) Test coverage added!
2. Good call, added comment.

Committed as a87e8aa and pushed to 8.5.x.

Cherrypicked as 3d0abda and pushed to 8.4.x.

Thanks @WimLeers et al