Refactor Gitaly tests folder structure

What does this MR do and why?

Describe in detail what your merge request does and why.

Moves Gitaly tests out of 3_create stage folder to 12_systems to reflect the current state of things specified in https://about.gitlab.com/handbook/product/categories/#gitaly-group.

Plus general refactoring to resolve any RuboCop offences in the moved files.

Part of https://gitlab.com/gitlab-org/quality/team-tasks/-/issues/1284

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