Add Testing & Verification as a subcategory under App Definition & Development

Hi Team, wanted to suggest adding **Testing & Verification** as a subcategory under App Definition & Development.

This feels like a distinct segment now, but many of these tools are still split across broader buckets today. A dedicated subcategory would make it easier to discover projects focused on testing, verification, performance, conformance, replay, and resilience checks. The current Landscape already includes good examples such as Microcks, Keploy, k6, Testkube, Fortio, Sonobuoy, Tracetest, Litmus. 

If this direction makes sense, I’d be happy to help create the subcategory.

Start by reviewing the current Landscape taxonomy and how categories are represented in this repository. The issue identifies App Definition & Development as the parent and lists example projects to consider. Done means a Testing & Verification subcategory is added there and the relevant projects can be discovered under it.

Written by the indexing model from the issue text.