libc: tests: allow fortified test cases to require root

[kevans]

Summary

An upcoming test will require root to create a jail with its own
domainname/hostname to avoid external requirements on the test runner
as we want to fetch them with valid and plausible sizes.

[markj]

lib/libc/tests/secure/generate-fortify-tests.lua

1077

It'd be a bit more future-proof to always generate the header, then only add the atf_tc_set_md_var() when needed.