Update framework testing infrastructure and standards

- Add phpt test runner and suite for app functions
- Introduce testing workflow to AGENT.md
- Add tests for cache, content, context, helpers, hooks, plugins,
  rendering
- Mount tests directory in dev container

devel/compose.yaml

@@ -9,6 +9,7 @@ services:
       - ../app:/var/www/app:z
       - ./apache/custom.conf:/etc/apache2/conf-available/custom.conf:z
       - ./apache/default.conf:/etc/apache2/sites-available/000-default.conf:z
+      - ./tests:/var/www/tests:z
     ports:
       - "8080:80"
     command: bash -c "a2enconf custom && a2enmod rewrite && apache2-foreground"