Add guidelines on unit-testing to the shell scripting style guide

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Description of the proposal

During the discussion of gitlab-ce#64016, several unit-testing frameworks for shell scripts were mentioned. We need to evaluate the need in such frameworks in our systems that make heavy use of shell scripts and add recommendations to our guidelines.

Checklist

• Prepare a merge request
• Mention the proposal in the next backend weekly call and the #backend channel to encourage contribution
• Proceed with the proposal once 50% of the maintainers have weighed in, and 80% of the votes are 👍
• Once approved, mention it again in the next backend weekly call and the #backend channel

/cc @gitlab-org/maintainers/rails-backend