Currently we only execute tests using the v4only mode:

This can cause our IPv6 based integration tests to go stale. We should investigate what is required to enable IPv6 on Circle CI and start running with ENVOY_IP_TEST_VERSIONS=all.

Looks like Circle CI does support IPv6 for local traffic:

https://circleci.com/docs/2.0/faq/#can-i-use-ipv6-in-my-tests