testing: fix potential problems in testFlowControlAccountCheck (#5105)
testing: fix potential problems in testFlowControlAccountCheck (#5105)


grpc/grpc-go,menghanl/grpc-go,menghanl/grpc-go,dfawley/grpc-go,stanley-cheung/grpc-go,stanley-cheung/grpc-go,dfawley/grpc-go,grpc/grpc-go