[ticket/11720] Do not call $captcha->validate if $captcha is not set

PHPBB3-11566 changed big parts of code. Unfortunately, a call to
$captcha->validate was added that is being called even if $captcha hasn't
been initialized. This change will fix this issue.

PHPBB3-11720
[ticket/11720] Add functional test for submitting report as user

The already existing functional tests were not ran as the filename was
missing the appended "_test".

PHPBB3-11720
Merge remote-tracking branch 'marc1706/ticket/11720-prep-release' into prep-release-3.0.12

* marc1706/ticket/11720-prep-release:
  [ticket/11720] Add functional test for submitting report as user
  [ticket/11720] Do not call $captcha->validate if $captcha is not set