[ticket/11528] Make sure to logout in tearDown

Teardown by default clears all cookies but since there is an
additional login inside this teardown(), logout needs to be used.

PHPBB3-11528
[ticket/11528] Use null for unchecked checkboxes

Default unchecked checkboxes do not have 'checked' attribute set.
Use null to assert instead of ''

PHPBB3-11528