Add test evidence for recent major changes (OpenSSF)

[adharsh277]

Add evidence of testing policy adherence for recent major changes

This PR updates the documentation at docs/testing-evidence.md to demonstrate Flatcar's adherence to its testing policy. The document presents concrete examples where automated tests were added alongside major feature changes, supporting the OpenSSF Best Practices Badge requirements for CNCF graduation.

The update includes not only previously cited versions (3888.0.0, 3815.2.1), but also more recent releases such as 4152.2.1. It captures PRs across multiple Flatcar components (e.g., bootengine, init, coreos-overlay), showing test coverage was considered during the introduction of new functionality.

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How to use

To validate this PR:

1. Open the file docs/testing-evidence.md
2. Review the listed Flatcar versions and their major feature changes
3. For each entry:
   • Visit the referenced PR(s)
   • Confirm the feature or config change was introduced
   • Check whether unit or integration tests were added in the same or subsequent PR

This supports evidence collection for OpenSSF compliance.

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Testing done

This was a documentation task with manual verification:

• Reviewed Flatcar release notes
• Identified major features added in versions such as:
  • 3888.0.0 (Ignition update)
  • 3815.2.1 (Containerd v1.7)
  • 4152.2.1 (Image signing key support, kernel config updates, Ignition behavior change)
• Located the corresponding PRs in the relevant Flatcar repositories (e.g., init, bootengine, coreos-overlay)
• Verified test files were added or modified where applicable
• Created a Markdown summary listing each feature, PR(s), and expected/verified test evidence

No build, CI, or functional tests were required.

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Checklist

• Changelog entries added
  ❌ Not required (this is a documentation-only update)

• Inspected CI output for system-level differences
  ✅ Not applicable (no changes to image or runtime behavior)