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.

(cherry picked from commit efba49ee5a64f8f56f49e58887484284d785d542)