[test] Use either `127.0.0.1` or `[::1]` to run in ipv6-only environments.

Test for both IPv4 and IPv6 support to determine if either `127.0.0.1` or `[::1]` are appropriate IP addresses to attempt to connect to. In an IPv6-only environment, `127.0.0.1` is not available.

Using `localhost` is problematic because we might not be able to get the same port on each IP flavor, and later try to connect to the wrong thing.

Reviewed By: labath

Differential Revision: https://reviews.llvm.org/D133393