CONTRIBUTING.md: define testing requirements for contributions

Part of #6393.

Problem

The Testing section of CONTRIBUTING.md is currently a single, aspirational sentence ("Testing is a key component in the development workflow.") with no actual guidance or requirement. doc/Developing.md explains how to run the unit tests but never states that contributions are expected to add or update tests.

Proposed change

Replace the stub Testing section in CONTRIBUTING.md with explicit guidance that:

• Fixes and features are expected to add or extend automated tests in the AppInstallerCLITests project (and E2E tests in AppInstallerCLIE2ETests where relevant).
• Describes at a high level how to build and run the tests, linking to doc/Developing.md ("Running Unit Tests").
• Notes that PRs without appropriate test coverage may be asked to add it before review completes.

Notes for engineers

Please confirm the exact bar we want to set: unit vs. E2E expectations, when tests can be reasonably omitted (e.g., pure doc changes), and any coverage guidance.

Acceptance criteria

• CONTRIBUTING.md Testing section states the expectation to add/update tests and links to how to run them.

(Created with GitHub Copilot assistance.)

Start with the Testing section in CONTRIBUTING.md, then read the “Running Unit Tests” section of doc/Developing.md and review the AppInstallerCLITests and AppInstallerCLIE2ETests references in the issue. Done means the CONTRIBUTING.md section states when contributions should add or update tests, links to the test-running guidance, and addresses the stated exceptions and expectations.