test(files_sharing): refactor EtagPropagationTest to avoid race, clarify setup, improve assertions

[joshtrichards]

• Resolves: #

Summary

• Share acceptance now occurs while logged in as the recipient user, ensuring mounts/resharing work reliably and eliminating race conditions.
• Clarified variable names for shares and nodes; replaced repeated $share usage with context-specific, descriptive names.
• Moved and expanded assertions: All relevant files/folders are now checked to exist after creation for every user context (user1–user4), making failures easier to diagnose.
• Logical grouping and ordering: Setup is now clearly sectioned per user (create, accept, organize, assert).
• This removes some test flakiness and delivers a more maintainable, realistic representation of user workflows.

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)