diff: Correctly declare tests

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)