Fix problems in authpdo

[splitbrain]

this fixes three problems:

• retrieveUsers() did only return user names instead of full user info authpdo: wrong return types of retrieveUsers and getUserCount in auth.php #1781
• getUserCount() returned a boolean instead of the count authpdo: wrong return types of retrieveUsers and getUserCount in auth.php #1781
• filters for groups were not handled correctly

Tests have been adjusted accordingly.

[mention-bot]

@splitbrain, thanks for your PR! By analyzing the history of the files in this pull request, we identified @phjanderson to be a potential reviewer.