StepCreateCD: Clean up temporary directory and add more robust tests

[saleemrashid]

This adds more robust tests that check the actual filenames and contents in the temporary directory, as well as removing the temporary directory in the step cleanup. This also tests adding folders, which wasn't previously being tested.

The motivation for checking the content is to allow tests for #61 that test how cd_files and cd_content interact (e.g. that cd_content correctly takes precedence over cd_files).

[azr]

Hello there, thanks for opening, this LGTM and you simply need to fix some linting issues: https://app.circleci.com/pipelines/github/hashicorp/packer-plugin-sdk/372/workflows/dd870b66-cf48-4d09-ba04-abdc676ba893/jobs/3054

Thanks for your time 🙂

[azr]

Fixed the linting issues !

[saleemrashid]

Thank you, and sorry I hadn't had a chance to fix them 🙏

[azr]

👍🏼 🙂 all good !