[ticket/13207] Use assert_checkbox_is_checked for registration test

PHPBB3-13207
Merge remote-tracking branch 'github-marc1706/ticket/13207' into develop-ascraeus

* github-marc1706/ticket/13207:
  [ticket/13207] Use assert_checkbox_is_checked for registration test
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/13207] Use assert_checkbox_is_checked for registration test