Assert only that the preview image gained no extra attributes

Pinning the exact attribute set also pinned which of them Trix's own
sanitizer keeps, which is not what this test is about.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0186eyzivcTn6wqjEE4Wnxdt

test/system/unfurling_links_test.rb

@@ -21,7 +21,7 @@ class UnfurlingLinksTest < ApplicationSystemTestCase
     assert_selector "trix-editor .og-embed__title", text: "A normal looking link"
 
     assert_equal @website.image_url, preview_image_attributes["src"]
-    assert_equal %w[ class src ], preview_image_attributes.keys.sort
+    assert_empty preview_image_attributes.keys - %w[ src class alt ]
   end
 
   private