agent/acs/handler: update attach eni handler tests to interact directly with the message channel instead of invoking handleSingleMessage
agent/acs/handler: update attach eni handler tests to interact directly with the message channel instead of invoking handleSingleMessage

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