Test migrateUserGroup.php

Why:

• The maintenance scripts in core are mostly untested and testing the less complex scripts will improve the test coverage in core.
• The migrateUserGroup.php maintenance script has a few bugs which were found by testing the script, and are fixed.

What:

• Add an integration test for migrateUserGroup.php
• Fix the maintenance script to:
  • Correctly report the number of affected users (instead of affected rows)
  • Clear the cache in the UserGroupManager when a batch is performed.

Bug: T371167
Change-Id: Ibd4c82332cbe83b7c5576a9e3bce6a99fa15c240