[ticket/11620] Rename class_functions_test -> extract_page_test

Renaming this file because it is going to contain a large data provider,
so I'd rather split this test out.

PHPBB3-11620
[ticket/11620] Add data provider to extract_page

These test cases were taken from a live session,
more test cases should be added to test specific
functionality in this function.

PHPBB3-11620