Fix stale test project references in CLAUDE.md

[harder]

Summary

The Build & Test section of CLAUDE.md referenced a Tests/UnitTests project that no longer exists. The actual test projects are UnitTestsParallelizable, UnitTests.NonParallelizable, and UnitTests.Legacy (per Tests/README.md). Copy-pasting the documented dotnet test command would fail.

• Replaced the stale Tests/UnitTests reference with the three current projects, each annotated with when to use it.
• Added a single-test --filter example.
• Added a pointer to Tests/README.md for the full list of test projects and the static-state classification that determines where a new test belongs.
• Updated the "Testing" summary bullet to reflect the three-project split and the rule that new tests never go in UnitTests.Legacy.

Docs-only change — no code, tests, or warnings affected.

Test plan

• CLAUDE.md renders correctly on GitHub
• The dotnet test --project Tests/UnitTestsParallelizable --no-build command in the doc matches a real project path
• No source files touched, so no new warnings possible

To pull down this PR locally:

git remote add copilot https://github.com/harder/Terminal.Gui.git
git fetch copilot docs/claude-md-fix-test-projects
git checkout copilot/docs/claude-md-fix-test-projects

🤖 Generated with Claude Code