Allow Quality::TestLevel to have several prefixes

What does this MR do and why?

Second attempt after !71308 (merged)got reverted. In the meantime, !71479 (merged) was merged to address the possibility of breaking master (tracked by #342022 (closed)).

By allowing Quality::TestLevel to take several prefixes, we can deduplicate the rspec frontend_fixture/rspec-ee frontend_fixture to generate the FOSS & the EE-specific fixtures in a single (parallelized) job in both FOSS and EE.

• In gitlab-foss, the job generates FOSS fixtures (since the ee/ folder isn't there).
• In gitlab, the job generates FOSS & EE fixtures (since it's looking at spec files under both spec/frontend/fixtures and ee/spec/frontend/fixtures).

I realized that we could optimize and simplify this job while working on !70000 (merged).

MR acceptance checklist

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