Add pytest-xvfb to test environment

.github/workflows/continuous-integration.yml

@@ -31,7 +31,7 @@ jobs:
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
-        pip install flake8 pytest pytest-cov pytest-qt ipython
+        pip install flake8 pytest pytest-cov pytest-qt pytest-xvfb ipython
     - name: Lint with flake8
       run: |
         # stop the build if there are Python syntax errors or undefined names