Extract test-test workflow with correct job prerequisites - #170

This PR addresses issue #169 by separating test tools validation into its own workflow job with the correct prerequisite structure.

Changes Made

Package Scripts

• New test-test script: Runs test tools validation (fixtures-html-validate-should-fail.mjs and fixtures-structured-data-should-fail.mjs)
• Updated test script: Now only runs application tests (build-html-validate.mjs, dirty-file-paths-checker.mjs, and build-structured-data-validate.mjs)

GitHub Actions Workflow Structure

• Consolidated workflow: All jobs are now in build-test-deploy.yml with proper dependencies
• Correct prerequisites:
  • Jekyll build is prerequisite for testing (test job depends on build)
  • Test-test is prerequisite for testing (test job depends on test-test)
  • Jekyll build is prerequisite for deploying (deploy-github-pages job depends on build)
  • No other prerequisites exist

Job Flow

build (Jekyll) ──┬──→ test (depends on build + test-test) 
                 │
test-test ───────┘
                 
build ──→ deploy-github-pages (depends only on build)

Benefits

• Improved CI/CD efficiency: Test tools validation runs in parallel with Jekyll build
• Clear separation of concerns: Test tool validation is separate from application testing
• Correct deployment gates: Deployment only requires successful build, allowing faster releases
• Better failure isolation: Test tool issues don't block deployment if build succeeds

Verification

Both test scripts work independently and the workflow has the correct prerequisite structure:

# Test tools validation
yarn run test-test

# Application tests  
yarn run test

The workflow ensures test-test validation completes before running main tests, while deployment only waits for the build to complete.

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