Force diff report_identical test to use unprivileged_user

[olivier]

Summary

Running the full regression suite is done using root user: diff is still able to read the unreadable (chmod -r) file if run with root.

Test Plan

Before, as root:

cd /usr/tests
kyua test usr.bin/diff/diff_test:report_identical

is failing (no permission error message generated by diff).
After applying this patch, diff correctly generate the error, so pass the test.