Fix spec/features/snippets/notes_on_personal_snippets_spec.rb

What does this MR do?

Fallback to default pattern when note does not belong to project

In commit 020295ff a regex was added to quickly determine if mentionables might be referenced. But this didn't take into account the note might not belong to a project. So when it doesn't belong to a project, assume the default issue tracker is used, and use DEFAULT_PATTERN to check if there are mentionables.

What are the relevant issue numbers?

Fixes gitlab-org/gitlab-ce#31993