[ticket/12398] Use submit_post return in prune_shadow_topic_test

This should get rid of possible failures during functional tests.

PHPBB3-12398
[ticket/12398] Use return of submit_post in softdelete and download tests

PHPBB3-12398
Merge remote-tracking branch 'marc/ticket/12398' into develop-ascraeus

* marc/ticket/12398:
  [ticket/12398] Use return of submit_post in softdelete and download tests
  [ticket/12398] Use submit_post return in prune_shadow_topic_test