Fixed watcher to also update the owner's folder sizes

Note that the root folder size is mandatory for quota calculation.
Removed unused method
Added unit tests for shared files watcher

- Added base class for sharing unit tests
- Added unit tests for shared files watched to check for folder size
  propagation
Fixed unit tests for files sharing watcher
Merge pull request #5701 from owncloud/sharing-updateownerfoldersizes

Fixed watcher to also update the owner's folder sizes