R1:5058cfb9726f

Pass a real viewer to HeraldAdapter when doing test console runs

Summary:
Depends on D18932. Ref T13048. See PHI276. In the cluster, we don't have device keys on `web` nodes. This is generally good, since they don't need them, and it means that we aren't putting more credentials than we need on those hosts.

However, it means that when we pull diff content to test "Commit" rules via the Herald test console, we use the omnipotent user and try to use device credentials, and this fails since we don't have…