feat: add lib/support/flattenMessage.js test codes

[tomfrombayesians]

Your checklist for a pull request to sitespeed.io

Please review the guidelines for contributing to this repository.

• I'm making a big change or adding functionality so I've already opened an issue proposing the change to other contributors, so I got feedback on the idea before took the time to write precious code
• Check that your change/fix has corresponding unit tests (if applicable)
• Squash commits so it looks sane
• Update the documentation https://github.com/sitespeedio/sitespeed.io/tree/main/docs in another PR
• Verify that the test works by running npm test and test linting by npm run lint

Description

Hello!

I’m excited to make my first contribution to the sitespeed.io. My team is developing an open source agent called Tom to assist with code maintenance. And I'm engineer, user of sitespeed. This code is the result of a collaboration between Tom and me, and our ultimate goal is to fully automate the maintaining workflow. Although it’s currently private, you can join the Beta Release Waitlist using this link.

---

I’ve carefully reviewed the link provided in the issue, and I found that the utility functions were the easiest for me to understand. Since the lib/support directory didn’t have any test code before, I’ve created a new test file. I wanted to confirm that my approach is aligned with your expectations before contributing further. Please let me know if any changes are needed.

Thank you so much for your time and consideration!