[ticket/11568] Use Goutte Client to install the board for functional tests

PHPBB3-11568
[ticket/11568] Set client manually so we can increase the cURL timeout

PHPBB3-11568
[ticket/11568] Any output before the doc type means there was an error

Change is needed to allow "Warning:" in the HTML body

PHPBB3-11568
[ticket/11568] Only use a static version of the client

PHPBB3-11568
[ticket/11568] Remove manual calls to assert_response_success()

PHPBB3-11568
[ticket/11568] Make CookieJar static aswell

PHPBB3-11568
[ticket/11568] Fix common_groups_test.php form handling

PHPBB3-11568