Various clean ups to Gitaly's tests and templating

What does this MR do?

This MR contains various clean ups to tests and templating logic of the Gitaly package. Mainly, it removes a bunch of regex assertions and replaces them with a equality assertion of the entire config. This ensures the entire configs content is asserted, not just what the regexes match. Likewise, this also removes the dependency on specific formatting of the config and rather ensures that the resulting TOML config is correct. This makes it easier to iterate later on the config generation in gitaly#4467 (closed).

Various test cases are also collapsed into the custom config values test. This eases maintenance by removing unnecessary test duplication.

Some unneeded templating logic around the rate limiting is also removed. Our aim is to remove the special templating logic in Omnibus and instead rely on the logic inside Gitaly.

This is a preparatory MR for gitaly#4467 (closed). I'll later open an MR that performs the same set of changes for Gitaly as !6373 (merged) performs for Praefect.

Related issues

gitaly#4467 (closed)

Checklist

See Definition of done.

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Expected (please provide an explanation if not completing)

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