Explicitly set group for repositories_storages and improve manage-storage-directories tests

What does this MR do?

Explicitly set group for repositories_storages such that /var/opt/gitlab/git-data/repositories is owned by git:git by default as called out at https://docs.gitlab.com/omnibus/settings/configuration.html#changing-the-name-of-the-git-user--group. Currently group is not set so it defaults to git:root based on parent ownership or could be something else if parent folder ownership gets messed up.

Helps address unknown NFS issue I had (gitlab!42774 (diffs, comment 416594050)) where chmod 2755 could not be run on folder owned by git:root.

Also

• Make tests use valid paths
• Update tests to check for related permissions set
• Improve manage-storage-directories tests and make more consistent

After merge action for myself:

• update https://docs.gitlab.com/ee/administration/nfs.html#testing-nfs to test chmod 2755 against git:git rather than git:root

Related issues

Checklist

See Definition of done.

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Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
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Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened