Enable branch coverage for SimpleCov

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Summary

We previously attempted this in !81504 (merged), but this caused a master:broken #354083 (closed) and we reverted the change in !81984 (merged).

Improvements

Reenable branch coverage, but also:

• Make sure we don't trigger the same failures from https://gitlab.com/gitlab-org/gitlab/-/pipelines/482132092/failures.
• Investigate runtime differences for the rspec:coverage job, we had at least one case where it was running for 47m: https://gitlab.com/gitlab-org/gitlab/-/jobs/2150725014.
• Also investigate runtime differences for the overall pipeline, I assume all RSpec jobs will run slower since they record additional information.