test: extend coverage for apply remote changes logic

Add test cases for apply remote changes logic.

- Verify correctness of db_version values from the cloudsync_changes virtual table.
- Test merge function with and without payload_apply_callback (the callback adds savepoints that may affect cloudsync_payload_apply transaction logic).
- Add test for correctness of LWW (last-write-wins) logic.
- Add test for merging changes from 10 clients across multiple operation rounds.

Author: andinux