[FAB-13411] fix flake in TestServerInterceptors

The server stream interceptor added by TestServerInterceptors returns an
error that causes the server side of the stream to terminate. That means
the client side send may get an io.EOF from Send if the termination of
the server side of the stream was detected.

Change-Id: I237f0aad6daa2878a987509e35074a16b689feba
Signed-off-by: Matthew Sykes <sykesmat@us.ibm.com>
(cherry picked from commit 6e56e6e)

core/comm/server_test.go

@@ -178,14 +178,18 @@ func invokeEmptyStream(address string, dialOptions []grpc.DialOption) (*testpb.E
 		}
 	}()
 
-	if err := stream.Send(new(testpb.Empty)); err != nil {
-		return nil, err
+	// TestServerInterceptors adds an interceptor that does not call the target
+	// StreamHandler and returns an error so Send can return with an io.EOF since
+	// the server side has already terminated. Whether or not we get an error
+	// depends on timing.
+	err = stream.Send(&testpb.Empty{})
+	if err != nil && err != io.EOF {
+		return nil, fmt.Errorf("stream send failed: %s", err)
 	}
 
 	stream.CloseSend()
 	<-waitc
 	return msg, streamErr
-
 }
 
 const (