[lib] Add unit tests for mention utils

Summary:
This diff introduces tests for lib/shared/mention-utils.js file.

Depends on D8908.

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

Reviewers: tomek, inka, rohan

Reviewed By: inka

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D9254