HTTPS Previews: Add IPv6 Test

There was a bug server side where encoding a IPv6 URL wasn't working
correctly. This is a preemptive test to make sure the encoding in Chrome
is correct (it already is).

Change-Id: I17dd2393133fc02f01de0b2ef381fc05067bf72d
Reviewed-on: https://chromium-review.googlesource.com/c/1336508
Commit-Queue: Robert Ogden <robertogden@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608168}