libc: tests: fix the gethostname() and getdomainname() tests

Instead of relying on any particular domainname and hostname to succeed,
spin up a jail before we execute the test with them set to some known,
fixed values. This allows them to be meaningfully tested -- previously,
they were skipped much more often than not.

Reported by: jlduran
Reviewed by: jlduran, markj
Differential Revision: https://reviews.freebsd.org/D49237