[ticket/11620] Remove unneeded functions from testable facade

There are functions listed in testable facade that don't have a lot of dependencies,
instead mostly just take the input and perform database functions on them.
These can be tested without a testable facade function and so will be removed.

PHPBB3-11620
[ticket/11620] Add validate_referrer test

Add a test for the validate_referrer function.

PHPBB3-11620