Install xvfb before building

Author: iluwatar

.github/workflows/maven.yml

@@ -20,5 +20,7 @@ jobs:
       uses: actions/setup-java@v1
       with:
         java-version: 11
+    - name: Install xvfb
+      run: sudo apt-get install xvfb
     - name: Build with Maven
       run: mvn clean verify