[ticket/11561] Specify used tables in notification fixture, so they are emptied

PHPBB3-11561
Merge remote-tracking branch 'nickvergessen/ticket/11561' into develop

# By Joas Schilling
# Via Joas Schilling
* nickvergessen/ticket/11561:
  [ticket/11561] Specify used tables in notification fixture, so they are emptied