client_lb_end2end_test: fix broken tests (#29938)
client_lb_end2end_test: fix broken tests (#29938)

* client_lb_end2end_test: fix broken tests

* poll polling engine gives slightly different error messages
grpc/grpc,jtattermusch/grpc,stanley-cheung/grpc,stanley-cheung/grpc,jtattermusch/grpc,jtattermusch/grpc,jtattermusch/grpc,grpc/grpc,ejona86/grpc,stanley-cheung/grpc,jtattermusch/grpc,ctiller/grpc,ctiller/grpc,ejona86/grpc,ctiller/grpc,ctiller/grpc,grpc/grpc,ejona86/grpc,grpc/grpc,ctiller/grpc,ejona86/grpc,ejona86/grpc,jtattermusch/grpc,stanley-cheung/grpc,stanley-cheung/grpc,grpc/grpc,stanley-cheung/grpc,ctiller/grpc,ctiller/grpc,ejona86/grpc,ejona86/grpc,ctiller/grpc,stanley-cheung/grpc,jtattermusch/grpc,grpc/grpc,ejona86/grpc,jtattermusch/grpc,jtattermusch/grpc,ctiller/grpc,jtattermusch/grpc,ejona86/grpc,stanley-cheung/grpc,ctiller/grpc,jtattermusch/grpc,jtattermusch/grpc,grpc/grpc,stanley-cheung/grpc,grpc/grpc,ctiller/grpc,grpc/grpc,ejona86/grpc,grpc/grpc,stanley-cheung/grpc,ejona86/grpc,stanley-cheung/grpc,ejona86/grpc,grpc/grpc,stanley-cheung/grpc,ctiller/grpc,grpc/grpc