[lib] Add unit tests for chat mention regex

Summary:
This diff adds chat mention regex unit tests to markdown.test.js.

Depends on D8834.

Test Plan: Run yarn test in lib folder and check if all tests pass.

Reviewers: tomek, inka, rohan

Reviewed By: rohan

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D9056