[BUGFIX] Do not try to cover an interface in tests (#660)

Only classes and traits can be covered by code coverage. Trying to
cover an interface causes PHPUnit to issue a warning.

Author: oliverklee

tests/Comment/CommentTest.php

@@ -10,7 +10,6 @@
 
 /**
  * @covers \Sabberworm\CSS\Comment\Comment
- * @covers \Sabberworm\CSS\Comment\Commentable
  * @covers \Sabberworm\CSS\OutputFormat
  * @covers \Sabberworm\CSS\OutputFormatter
  */