Add extension lang files to extension functional test case

This should allow extensions to load their own lang files in functional testing, and use this->lang() instead of hardcoding language into their tests, e.g.:
```
$this->add_lang_ext(array('boardrules_common', 'boardrules_controller');

$this->assertContains($this->lang('BOARDRULES_HEADER'), $crawler->text());
$this->assertContains($this->lang('BOARDRULES'), $crawler->text());
```
This is based off the add_lang() function from the phpbb_functional_test_case