autotest: WiFiManager: Test modifying Service by GUID

In addition to the prvious GUID test where we used
ConfigureService to create services by GUID, add a test
which uses ConfigureService to change the passphrase
on an already-configured WiFi service.

BUG=chromium-os:34004
TEST=Rerun test
CQ-DEPENDS=Ie23c1b03dee89aea54a4a86c46734165488a885b

Change-Id: Ibc9c8ff7ed929adbd8b9d9c9b56a3c860b6636a0
Reviewed-on: https://gerrit.chromium.org/gerrit/32970
Reviewed-by: Wade Guthrie <wdg@chromium.org>
Reviewed-by: mukesh agrawal <quiche@chromium.org>
Commit-Ready: Paul Stewart <pstew@chromium.org>
Tested-by: Paul Stewart <pstew@chromium.org>
1 file changed