Functional Test Framework should include functions.php

Functional tests run by extensions will fail if they have not already loaded functions.php themselves elsewhere in their suite.

This is because when the functional test framework sets up functional testing, it needs functions.php available, because of this line in the new installer, calling get_preg_expression()
https://github.com/phpbb/phpbb/blob/3.2.x/phpBB/phpbb/install/module/obtain_data/task/obtain_admin_data.php#L194

We should probably require/include functions.php in the functional test framework by default, so the framework works again, as expected.