fix: check instance of storage using helper function

instanceof cannot be used to check the instance of a storage, doing so
breaks the check in certain cases. In this case, enabling the
`files_accesscontrol` app breaks the check.

Signed-off-by: Salvatore Martire <4652631+salmart-dev@users.noreply.github.com>

apps/dav/tests/unit/Connector/Sabre/DirectoryTest.php

@@ -19,6 +19,7 @@ use OCP\Constants;
 use OCP\Files\ForbiddenException;
 use OCP\Files\InvalidPathException;
 use OCP\Files\Mount\IMountPoint;
+use OCP\Files\Storage\IStorage;
 use OCP\Files\StorageNotAvailableException;
 use Test\Traits\UserTrait;