Stack: Validating generated build artifacts option on tests

[pacostas]

Summary

This PR extends the functionality of test.sh script on stacks, to validate the generated artifacts before testing.
More specifically, if the option --validate-stack-builds is specified and the build artifacts are not generated or are not generated properly, then the script will throw an error.
The default behavior has not been changed, so in case the option --validate-stack-builds option is not specified, and some builds are missing, it will regenerate them.

Also, on this PR, it has been changed on where to look for the images.json file, as the default location has changed.

Use Cases

The --validate-stack-builds option is useful on the create release workflow, where we want to ensure that the generated artifacts that are generated from previous steps, are not missing. So, in case of missing artifacts, the script will not regenerate them, instead, it will throw an error.

Checklist

• I have viewed, signed, and submitted the Contributor License Agreement.
• I have linked issue(s) that this PR should close using keywords or the Github UI (See docs)
• I have added an integration test, if necessary.
• I have reviewed the styleguide for guidance on my code quality.
• I'm happy with the commit history on this PR (I have rebased/squashed as needed).