test(files): stabilize CachePermissionsMask tests

[joshtrichards]

• Resolves: (transient test/CI failures) #

Summary

Remove the pre-created foo folder from CachePermissionsMaskTest::setUp() so inherited CacheTest cases still start with the default cache contents.

The test class intentionally changes the cache wrapper in setUp(), but storage fixtures should be (and already are) created in the individual tests that need them.

Also tighten permission masking coverage by:

• adding explicit coverage for direct get() on scanned file and folder entries
• verifying scan_permissions is populated from original permissions
• verifying existing scan_permissions values are not overwritten

This fixes a transient failure in inherited cache tests¹ while keeping the wrapper-specific behavior covered and closing coverage gaps.

The final commit only cleans up and modernizes the test class.

TODO

• Backport to at least v34

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

Footnotes

1. Example CI failure: https://github.com/nextcloud/server/actions/runs/28551681637/job/84737184657?pr=61707 ↩