Samples for terraform testing

Adding samples and best practices for terraform testing.
Including the full spectrum of tooling:
- terrastest
- terraform-complicance
- inspec-iggy

No files, tests, or entry points are named. Start by reviewing the requested Terraform testing samples and the listed tools—Terratest, terraform-compliance, and inspec-iggy—and define the sample and best-practice scope before editing; done means the requested testing guidance and examples are documented.

Written by the indexing model from the issue text.