update_handler: Clean up test assertions
update_handler: Clean up test assertions

Testify's assertions clean this up considerably. The require.* funcs
are needed to stop the test on assert failures. assert.* funcs are
used only where the test can continue regardless of the assertion,
despite the test being marked as a FAILure.

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