diff: Correctly declare tests

Approved by: re (cperciva)
Sponsored by: Klara, Inc.
Fixes: 5fc739eb5949 ("diff: Fix integer overflows in Stone algorithm")
Fixes: 270492602b9b ("diff: Add test case for pagination resource leak")
Fixes: 590126789c84 ("diff: Don't compare a file or directory to itself")

(cherry picked from commit 157d6664aeb815db3b758bd3038fd1512a0f4e2c)
(cherry picked from commit b8bcf04d6dd37308834ee258230082ae6875b1f3)