libc: tests: allow fortified test cases to require root

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.

Generate test headers for all cases to reduce churn in future diffs as
metadata is added to individual tests, or in case other test options
are added to correspond to different metadata to set.

Reviewed by: jlduran, markj
Differential Revision: https://reviews.freebsd.org/D49236