Add e2e test of repo usage statistics

What does this MR do and why?

Adds an end-to-end test to verify that the repository usage statistics reports a size that matches one that a user could calculate from the same repo.

Closes gitlab-org/quality/testcases#3178

Screenshots or screen recordings

How to set up and validate locally

bundle exec bin/qa Test::Instance::All http://gitlab.url -- qa/specs/features/api/3_create/repository/storage_size_spec.rb

MR acceptance checklist

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