Avoid the DNS lookup for "localhost"

ci.FreeBSD.org does not have access to a DNS resolver/network (unlike my test
VM), so in order for the test to pass on the host, it needs to avoid the DNS
lookup by using the numeric host address representation.

PR: 235200
Reviewed by: asomers, lwhsu
Approved by: emaste (mentor)
MFC after: 2 weeks
MFC with: r343362, r343365, r343367-r343368, r343461
Differential Revision: https://reviews.freebsd.org/D19026