Speedup share tests

[icewind1991]

• Resolves: #

Summary

• Mock OCM discovery
• Better dummy user/group backend usage
• don't trigger cache gc during tests
• reduce number of filesystem setups

Reduces time to run the share tests by ~2/3 in my testing

TODO

• ...

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI