signer tests: Allow different appids for canary channel images

If the channel is canary, allow appid to match the value of
expected_appid_canary in the ensure sane lsb release test
configuration.

BUG=chromium-os:25437
TEST=manually tested on an image with and without the channel being canary.

Change-Id: I6bf71adbe0fc090ef777c28d24c53eaa8be18404
Reviewed-on: https://gerrit.chromium.org/gerrit/15509
Tested-by: Gaurav Shah <gauravsh@chromium.org>
Reviewed-by: Scott Zawalski <scottz@chromium.org>
Commit-Ready: Gaurav Shah <gauravsh@chromium.org>
2 files changed