client_lb_end2end_test: fix flake caused by not doing graceful server shutdown (#30647)
client_lb_end2end_test: fix flake caused by not doing graceful server shutdown (#30647)


stanley-cheung/grpc,jtattermusch/grpc,grpc/grpc,jtattermusch/grpc,jtattermusch/grpc,grpc/grpc,stanley-cheung/grpc,stanley-cheung/grpc,jtattermusch/grpc,stanley-cheung/grpc,grpc/grpc,grpc/grpc,stanley-cheung/grpc,jtattermusch/grpc,jtattermusch/grpc,stanley-cheung/grpc,stanley-cheung/grpc,grpc/grpc,jtattermusch/grpc,grpc/grpc,stanley-cheung/grpc,grpc/grpc,jtattermusch/grpc,jtattermusch/grpc,grpc/grpc,grpc/grpc,jtattermusch/grpc,stanley-cheung/grpc,stanley-cheung/grpc,grpc/grpc,grpc/grpc,stanley-cheung/grpc,stanley-cheung/grpc,grpc/grpc,jtattermusch/grpc,jtattermusch/grpc