Update the testing guide

[cqxswbc2]

Summary of Changes

• update the test-suite overview and commands for the current uv-based workflow
• correct the documented test layout and control-data paths
• explain single-frame and multi-frame graphical tests, control-data generation, and video metadata comparison tests
• copyedit the guide for clarity and consistency

Closes #2502.

Changelog

Testing

• git diff --check
• strict reStructuredText parse with Docutils 0.21.2
• codespell -L medias,nam docs/source/contributing/testing.rst

A full local Sphinx build was not completed because dependency downloads timed out; CI can provide the final documentation build result.

Checklist

• I have read the Contributing Guidelines
• I have written a descriptive PR title (see top of PR template for examples)
• My new documentation builds, looks correctly formatted, and adds no additional build warnings

Reviewer Checklist

• The PR title is descriptive enough
• The PR is labeled appropriately
• Newly added documentation builds, looks correctly formatted, and adds no additional build warnings