[ticket/12822] Correctly call \phpbb\user in avatar manager test

PHPBB3-12822
Merge remote-tracking branch 'github-marc1706/ticket/12822' into develop-ascraeus

* github-marc1706/ticket/12822:
  [ticket/12822] Correctly call \phpbb\user in avatar manager test
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/12822] Correctly call \phpbb\user in avatar manager test