Additional integration tests for digest resolution changes

[amogh09]

Summary

This PR adds a couple of additional integration tests for changes to enable image digest resolution in ECS Agent. The tests are -

• TestInvalidImageInteg - tests that a task with a single container that has an invalid image fails with a CannotPullImageManifestError error.
• TestImageWithDigestInteg - tests that a task with a single container whose image has a valid digest specified works as expected.

The PR also includes a change to fix flakiness of TestManifestPulledDoesNotDependOnContainerOrdering integration test. The test is flaky because it assumes that public.ecr.aws/docker/library/busybox:1.36.1 image is immutable but it's not. The PR also changes the test to use an image from local test registry instead as using a remote image does not affect the test.

Some minor refactoring changes and a typo fix is also included in this PR.

Testing

New tests cover the changes: yes

Description for the changelog

Enhancement: Additional integration tests for digest resolution changes

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.