interop-testing: fix bug of not completing client configure RPC in xDS test client (#7597)

Author: voidzcy

interop-testing/src/main/java/io/grpc/testing/integration/XdsTestClient.java

@@ -430,6 +430,8 @@ public void configure(ClientConfigureRequest request,
         newMetadata.put(metadata.getType(), md);
       }
       rpcConfig = new RpcConfig(request.getTypesList(), newMetadata);
+      responseObserver.onNext(ClientConfigureResponse.getDefaultInstance());
+      responseObserver.onCompleted();
     }
   }