Add integration tests to the Horizon check gate

Now that the job passes on master, time to verify its stability by
running it on all incoming patchsets. The job is already marked as
non-voting, so just move it to the check queue for now and make sure it
doesn't run on any of the stable branches.

Change-Id: I09a1c18cf1fce7f1e981b0b2ec68456729ba16b7
Merge "Add integration tests to the Horizon check gate"