[ticket/11528] Refactor install_board() code into a new class

PHPBB3-11528
[ticket/11528] Instantiate $client, $driver from functional test case

Make mink test case an abstract class, instantiate $driver, $client
from its child class i.e. functional test case.

PHPBB3-11528
[ticket/11528] Call parent::tearDown in overrided tearDown method

PHPBB3-11528
[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