Speed up elastic_rake_spec

What does this MR do and why?

This MR speed-ups ee/spec/tasks/gitlab/elastic_rake_spec.rb by 14 minutes.

Before

Finished in 41 minutes 1 second (files took 1.57 seconds to load)

After

Finished in 26 minutes 48 seconds (files took 1.59 seconds to load)

I have some ideas for future improvements, but I think this one is pretty good first iteration.

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