[CE] Extract EE specific code for spec/support

Backport for https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9724.

What does this MR do?

TODO:

• Extract the diff from spec/support/shared_examples/mentionable_shared_examples.rb

What are the relevant issue numbers?

Part of https://gitlab.com/gitlab-org/gitlab-ee/issues/9506.

Does this MR meet the acceptance criteria?

• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Security reports checked/validated by reviewer