[ticket/13207] Use assert_checkbox_is_checked for registration test

PHPBB3-13207
[ticket/13216] Use gmdate() instead of date() in datetime tests

PHPBB3-13216
[develop-ascraeus] Update version number to 3.1.1-RC1-dev
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 pull request #3067 from marc1706/ticket/13216

[ticket/13216] Use gmdate() instead of date() in datetime tests