Clean up Gitaly tests

What does this MR do?

• Add UTF-8 test and fix for GitalyClient::Ref#find_ref_name
• Remove "when gitaly is enabled" contexts: Gitaly is always enabled in testing
• Add a 'be_utf8' Rspec string matcher

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

Why was this MR needed?

Maintenance.

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
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

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