tests: fix PHPUnit @requires for APCU
@requires is an annotation used by PHPUnit to flag a test as requiring a
PHP function or extension.
c214c9a95 changed it to list two functions
separated by a slash:
@requires function apc_store/apcu_store
However that is not recognized by PHPUnit and the test is always
skipped.
Change the @requires to require the APCU extension.
Repro case:
phpunit.php --verbose includes/libs/MemoizedCallableTest.php
Follow-up
c214c9a95255abf1032d389d0fe8bcbde38b55db
Bug: T140587
Change-Id: I1fdd52c8fc8ca1c7c0c7f914e6da8774799ba5d8