Stub parallel spec file list - attempt to reproduce issue 432145

What does this MR do and why?

An attempt to reproduce https://gitlab.com/gitlab-org/gitlab/-/jobs/5555070910

I have also documented how to reproduce CI failures like this in Document how to reproduce RSpec job failures in CI (!137926 - merged)

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Before	After

How to set up and validate locally

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MR acceptance checklist

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