Add extension lang files to extension functional test case

[iMattPro]

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

[EXreaction]

Should return after this or you'll have an error.

[iMattPro]

It actually doesn't error :)

In the case of passing along an array, lang_path simply won't exist, so no include will be attempted, and an empty array would be merged in to the existing filled lang array.

This is an exact copy of the add_lang() function used in the main phpbb functional test case, only difference being the $lang_path here points to the extension's language files instead of phpBB's language files.