Fix CI: Install subversion for WordPress test setup

The CI was failing because the bin/install-wp-tests.sh script requires
svn command to checkout WordPress test files, but subversion is not
installed by default in GitHub Actions Ubuntu environment.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: hideack

.github/workflows/php.yml

@@ -52,5 +52,8 @@ jobs:
 
       - run: composer install
 
+      - name: Install subversion
+        run: sudo apt-get update && sudo apt-get install -y subversion
+
       - run: bin/install-wp-tests.sh test root root-password 127.0.0.1:${{ job.services.mysql.ports['3306'] }}
       - run: ./vendor/bin/phpunit