Add coverage data from decomposed jobs

What does this MR do and why?

Fixes undercoverage job failing where code only runs on decomposed databases

Fixes problem first identified in !75064 (comment 769835754)

Screenshots or screen recordings

You can see now in https://gitlab.com/gitlab-org/gitlab/-/jobs/1877715664 the decomposed jobs are now processed in the rspec:coverage job. It didn't previously

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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