Doublecheck CustomElementsRenderMarkupTest results

Per https://git.drupalcode.org/project/custom_elements/-/merge_requests/84#n..., a HTML wrapping element just disappeared from the test results in CustomElementsRenderMarkupTest:

 <drupal-media-image-full type="image" view-mode="full" slot="teaser-media">

Contents of the MR xomment:

The wrapping drupal-media element is gone here - I've no idea how the renaming causes that. But since the test does not pre-configure the article display I'm not sure we need to care. I'd be fine to add a follow-up to pre-configure the node article display in the thunder module + improve the test for it then.

So:

• Do that. (Note I think the article display does not belong in the thunder module, but I don't want to go into it now.)
• There's a @todo in the code Remove this (override of saved config) along with the dependency of custom_elements_test_paragraphs on custom_elements_thunder. Probably also do that. (Note the dependency is implicit; see comments on top.
• Maybe convert to a kernel test. This single test is the only one that isn't converted yet, and it's likely because of the usage of the file system -- which I didn't want to investigate when I was converting the others. (Maybe there's a file_test module we can use for the File::create() call, instead of copying an actual file? I don't know, I'm still learning.)