diff: tests: loosen up requirements for report_identical

This test cannot run without an unprivileged_user being specified
anyways, so just run as the unprivileged user. Revoking read permisions
works just as well if you're guaranteed non-root.

Reviewed by: pstef
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D34950