Test policy evidence

[adharsh277]

Test policy evidence

This PR updates the documentation under docs/testing-evidence.md to demonstrate that Flatcar adheres to its testing policy when implementing major changes. The document lists specific releases (e.g., 3888.0.0, 3815.2.1, and 4152.2.1) where key features were introduced and automated tests were added alongside those changes. This supports our compliance with the OpenSSF Best Practices Badge required for CNCF graduation.

How to use

• Open the file docs/testing-evidence.md.
• Review the listed major Flatcar versions and the corresponding feature changes.
• For each entry:
  • Visit the referenced feature and test pull requests.
  • Verify that the major functionality was introduced.
  • Confirm that tests (unit or integration) were added alongside or shortly after the feature change.

This validation process supports the evidence collection for OpenSSF compliance.

Testing done

This documentation update was manually validated by:

• Reviewing the Flatcar release notes to identify major features in versions 3888.0.0, 3815.2.1, and 4152.2.1.

• Locating the corresponding feature and test pull requests in the relevant Flatcar repositories (e.g., bootengine, init, coreos-overlay).

• Confirming that automated tests were added or modified for the new features.

• Creating this Markdown summary to document the evidence.

• Changelog entries added in the respective changelog/ directory
  ❌ Not applicable (this is a documentation-only update)

• Inspected CI output for image differences: /boot and /usr size, packages, missing binaries, kernel modules, etc.
  ✅ Not applicable (this change does not affect the actual image or runtime behavior)