[PHPBB3-11539] Add unit tests for several functions in functions.php

Several functions are not unit tested yet.

Progress

1.	Add unit tests for (phpbb_)is_absolute()	Unverified Fix	Joas Schilling [X] (Inactive)
2.	Add unit tests for style_select() in functions.php	Unverified Fix	Joas Schilling [X] (Inactive)
3.	Add unit tests for language_select() in functions.php	Unverified Fix	Joas Schilling [X] (Inactive)
4.	Add unit tests for obtain online functions in functions.php	Unverified Fix	Joas Schilling [X] (Inactive)