Install graphviz for tests

Author: jaapio

.github/workflows/push.yml

@@ -55,6 +55,10 @@ jobs:
       - phpunit-with-coverage
     steps:
       - uses: actions/checkout@v2
+
+      - name: Install graphviz
+        run: sudo apt-get update && sudo apt-get install -y graphviz
+
       - name: Setup PHP
         uses: shivammathur/setup-php@v2
         with: