Fix EZP-22958: invalid values in subtree criterion handler

JIRA: https://jira.ez.no/browse/EZP-22958

Add proper validation of subtree format (`/1/2/3/`) for all elements in the array $value passed to the Subtree criterion.

Note that this does force the pathString to end with a forward-slash (`/`).
Limitation values is *not* documented to accept a string, see: https://github.com/ezsystems/ezpublish-kernel/blob/master/eZ/Publish/API/Repository/Values/User/Limitation.php