Fix gitaly-ruby bundle poisoning in CI

What does this MR do?

Fixes the way we launch gitaly in CI, to prevent gitaly-ruby from running with gitlab-ce's Gemfile.

• takes more control of environment variables used for gitaly during testing
• stops using shared gem pool between gitlab-ce and gitaly (gitaly's gems kept being removed from the pool)
• test bundle integrity during 'test build' and 'test spawn' so that things fail early when they're broken
• try booting gitaly during setup-test-env, for quicker failures
• abort actual test jobs earlier if gitaly cannot be reached after boot

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

The bundle environment of gitaly-ruby was being poisoned by the bundle environment of gitlab-ce. Sometimes, gitaly-ruby would still boot and function in spite of this poisoning. Sometimes it would fail to boot and this would then make lots of tests fail with mysterious "Deadline exceeded" errors.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes gitaly#1184 (closed) and gitaly#1169 (closed)