Add tests to ensure translated content works as expected

Similar to #2560787-3: Add tests for entity access.

#2328659: Convert all existing entity_embed tests from WebTestBase to BrowserTestBase landed, unblocking this!

Here's a basic, albeit sloppy, test. There is still a lot of clean-up needed, and I'd love to know what edge cases need to be covered as well.

Yay, thanks for getting this started! Will review tomorrow.

Basically only nits!

1. +++ b/tests/src/Functional/EntityEmbedFilterTest.php
   @@ -195,4 +199,61 @@ class EntityEmbedFilterTest extends EntityEmbedTestBase {
   +    $node = $this->drupalCreateNode([
   

   s/$node/$host_entity/

2. +++ b/tests/src/Functional/EntityEmbedFilterTest.php
   @@ -195,4 +199,61 @@ class EntityEmbedFilterTest extends EntityEmbedTestBase {
   +    $translation = $node->addTranslation('pt-br')
   

   s/$translation/$host_entity_translation/

3. +++ b/tests/src/Functional/EntityEmbedFilterTest.php
   @@ -195,4 +199,61 @@ class EntityEmbedFilterTest extends EntityEmbedTestBase {
   +//    $url = $translation->toUrl('canonical', [
   +//      'language' => $translation->language(),
   +//    ]);
   +    // @todo This is a cheat. Figure out how to honestly generate the URL of
   +    // the translation.
   +    $this->drupalGet('/pt-br/node/' . $node->id());
   

   This is a test. Tests should be as simple and readable as possible. Using APIs to compute the URL is the wrong thing to do here. Hardcoding the expected URL is perfectly appropriate.

   So we can remove the 3 commented lines and the todo.

4. +++ b/tests/src/Functional/EntityEmbedFilterTest.php
   @@ -195,4 +199,61 @@ class EntityEmbedFilterTest extends EntityEmbedTestBase {
   +    // The embedded node does not have a Portuguese translation, so it should
   +    // display in English.
   +    $this->assertSession()->pageTextContains($this->node->getTitle());
   

   👍

5. +++ b/tests/src/Functional/EntityEmbedFilterTest.php
   @@ -195,4 +199,61 @@ class EntityEmbedFilterTest extends EntityEmbedTestBase {
   +    // Now that the embedded node is translated into Portuguese, it should be
   +    // displayed in Portuguese.
   +    $this->assertSession()->pageTextContains($this->node->getTranslation('pt-br')->getTitle());
   

   This also proves that no explicit cache invalidation was necessary; thereby confirming that Entity Embed already is bubbling the appropriate cacheability! 🎉

6. +++ b/tests/src/Functional/EntityEmbedFilterTest.php
   @@ -195,4 +199,61 @@ class EntityEmbedFilterTest extends EntityEmbedTestBase {
   +    $translation->body->value = '<drupal-entity data-entity-type="node" data-entity-uuid="' . $this->node->uuid() . '" data-entity-embed-display="entity_reference:entity_reference_label" data-entity-embed-settings=\'{"link":"0"}\' data-align="left" data-caption="test caption" data-langcode="' . $node->language()->getId() . '">This placeholder should not be rendered.</drupal-entity>';
   

   This is missing a comment. Something like:
   // Now make the Portuguese translation of the host entity use the data-langcode attribute to refer to a specific translation of the embedded entity.

Nits addressed.

+1 on the cleanup. Thanks!

We missed one aspect here: if the translation for the specified data-langcode does not exist. #3027445: Add tests to ensure `data-langcode` works when that translation does not exist; should fall back to default translation was filed for that edge case.