test.support.HOST should be "localhost" as it was in the past. See the bpo-29639.

Tests that need the IP address should use HOSTv4 (added) or the existing HOSTv6 constant.

This changes the definition and fixes tests that needed updating to deal with HOST being
the hostname rather than the hardcoded IP address.

This is only the first step in addressing https://bugs.python.org/issue29639.