Follow-up from "Show correct warning on issue when project is archived"

Add tests for discussion_locked_widget.vue component.

The following discussion from !20078 (merged) should be addressed:

• @winh started a discussion: (+1 comment)

  @acroitor Sorry for the delay—the changes look good to me. 👍

  I wonder if we should add unit tests for discussion_locked_widget.vue though.