agent/app: fix flaky TestDoStartTaskENIHappyPath test.
agent/app: fix flaky TestDoStartTaskENIHappyPath test.

Move the async call to StartImageCleanupProcess outside of the ordered expects

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