Add test coverage for .com only cronjob settings

What does this MR do and why?

Adds test coverage to confirm Gitlab.com-only cron jobs being added to Settings.

This will hopefully add confidence when changing those settings and help to avoid incidents such as gitlab-com/gl-infra/production#8378 (closed)

Screenshots or screen recordings

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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