Fix TestServerSource race condition (#15799)
Fix TestServerSource race condition (#15799)

If we set the desired error after we start the stream, the stream could
have a real error before we get to setting the error.
istio/istio,smawson/istio,geeknoid/istio,istio/istio,geeknoid/istio,istio/istio,smawson/istio,geeknoid/istio,geeknoid/istio,geeknoid/istio,smawson/istio,smawson/istio,geeknoid/istio,istio/istio,smawson/istio,smawson/istio,istio/istio