autotest: Create IPv6 client test

As opposed to the IPv6 Manager WiFi autotest, this test is client
side only, and uses a pseudo-ethernet interface pair to test whether
the client successfully negotiates the correct forms of IPv6 addresses.

BUG=chromium-os:39529
TEST=Run test

Change-Id: Ia6789187b360e7f4efb24be0cbffba49fec2b6a0
Reviewed-on: https://gerrit.chromium.org/gerrit/44663
Reviewed-by: Christopher Wiley <wiley@chromium.org>
Commit-Queue: Paul Stewart <pstew@chromium.org>
Tested-by: Paul Stewart <pstew@chromium.org>
3 files changed