Tests for sync algorithm

[sunkup]

The sync algorithm is prone to be refactored again soon, but we should still write 2-5 (simple if possible) tests for the sync algorithm in abstract Syncer.

Notes:

• If needed, make "sync()" internal/protected and "VisibleForTesting"
• It may be nice to divide the sync algorithm further into 4 methods, which can be tested individually

The tests should ideally cover:

• local collection delete - when remote collection is gone
• local collection update - when remote collection has changed
• local collection creation -when new remote collection was found
• sync of a local collections contents (local resource) - when (number of) collection entries changed

Depends on #896

[github-actions]

This PR/issue depends on:

• Generalize Syncer #896