chore(bugbot): Add testing conventions code review rules

[github-actions]

Note

The pull request "chore(bugbot): Add testing conventions code review rules" was created by @Lms24 but did not reference an issue. Therefore this issue was created for better visibility in external tools like Linear.

This PR adds some rules to bugbot's rulest to flag some testing-related issues we'd like to avoid.

Happy to add/remove/change rules as reviewers see fit.

[linear]

JS-1271