ecsclient: fix retry_handler_test.go
ecsclient: fix retry_handler_test.go

The test depended on an implementation detail of the sdk retry logic
internals. We were able to change the method here in order to better
detect the retry logic.

swipely/amazon-ecs-agent,swipely/amazon-ecs-agent,swipely/amazon-ecs-agent,kiranmeduri/amazon-ecs-agent,kiranmeduri/amazon-ecs-agent,aws/amazon-ecs-agent,aws/amazon-ecs-agent,aws/amazon-ecs-agent,swipely/amazon-ecs-agent,aws/amazon-ecs-agent,kiranmeduri/amazon-ecs-agent,kiranmeduri/amazon-ecs-agent