Create 0006-testing-harness-approach.md

[Rene2mt]

Committer Notes

Document architectural decision record (ADR) around development and selection of a testing harness approach & supporting testing framework for the FedRAMP automation project.

All Submissions:

• Have you selected the correct base branch per Contributing guidance?
• Have you set "Allow edits and access to secrets by maintainers"?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you squashed any non-relevant commits and commit messages? [instructions]
• Have you added an explanation of what your changes do and why you'd like us to include them?
• If applicable, have all FedRAMP Documents Related to OSCAL Adoption affected by the changes in this issue have been updated.?
• If applicable, does this PR reference the issue it addresses and explain how it addresses the issue?

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