[FEATURE] Add userGroups to backendUser model

This patch adds the new BackendUserGroup model as property to the new
BackendUser model. The patch adds the usual methods for a
SplObjectStorage and the corresponding unit tests

Change-Id: I047361810eeb48c53483c99ed0a79966d0dd0e00
Resolves: #39329
Releases: 6.1
Reviewed-on: https://review.typo3.org/13879
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Tymoteusz Motylewski
Tested-by: Tymoteusz Motylewski