tests: fix PHPUnit @requires for APCU

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