diff: Add test case for pagination resource leak

The pagination code leaks either processes or descriptors or both,
depending on the exact version of the code you have. Add a test case
which exercises this leak to facilitate fixing it.

Approved by: re (cperciva)
MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: thj, kevans
Differential Revision: https://reviews.freebsd.org/D55111

(cherry picked from commit 270492602b9bd8b8fce4f021f055804978bf3f23)
(cherry picked from commit 51c2384ce049ec92e07901cf8a84c171e5d72104)