Fix TestExecCommandAgent flakiness on AL1

[amogh09]

Summary

TestExecCommandAgent has been quite flaky on AL1. The test starts a /kill process inside the test container to kill a running ExecCommandAgent process. However, it does not wait for /kill to finish before checking that ExecCommandAgent has been killed causing the test to fail with the error below.

--- FAIL: TestExecCommandAgent (1.62s)
    engine_sudo_linux_integ_test.go:740: 
        	Error Trace:	/opt/amazon-ecs-agent/go/src/github.com/aws/amazon-ecs-agent/agent/engine/engine_sudo_linux_integ_test.go:740
        	            				/opt/amazon-ecs-agent/go/src/github.com/aws/amazon-ecs-agent/agent/engine/engine_sudo_linux_integ_test.go:681
        	            				/opt/amazon-ecs-agent/go/src/github.com/aws/amazon-ecs-agent/agent/engine/engine_sudo_linux_integ_test.go:454
        	Error:      	Not equal: 
        	            	expected: false
        	            	actual  : true
        	Test:       	TestExecCommandAgent
        	Messages:   	SSM agent was not found in container's process list

This PR fixes this issue by making the test verify that /kill process has finished before verifying that ExecCommandAgent process has been killed.

Testing

Reproduced the test failure locally. Ran the test many times after making the change in this PR. It did not fail.

New tests cover the changes: no

Description for the changelog

Bugfix: Fix flakiness of TestExecCommandAgent

Does this PR include breaking model changes? If so, Have you added transformation functions?

Licensing

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.