Don't break testing when runnins as root

The rehash test broke the test if run by root.  Instead, just skip the
check that requires non-root to be worth it.

Fixes #4387

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5184)

(cherry picked from commit 98ade242)