.github/workflows/tests.yml: add dependencies for GUI testing

According to pytest-qt's documentation [1]:
> It might happen that your tox run finishes abruptly without any useful information, e.g.:
> ...
> pytest-qt needs a DISPLAY to run, otherwise Qt calls abort() and the process crashes immediately.

To mitigate this, I've followed instructions in [2].

[1] https://pytest-qt.readthedocs.io/en/latest/troubleshooting.html#tox-invocationerror-without-further-information
[2] https://pytest-qt.readthedocs.io/en/latest/troubleshooting.html#github-actions-azure-pipelines-travis-ci-and-gitlab-ci-cd

.github/workflows/tests.yml

@@ -13,7 +13,8 @@ jobs:
         os : [ubuntu-latest]
         python-version: ["3.8", "3.9", "3.10", "3.11", "3.12"]
       fail-fast: false
-
+    env:
+      DISPLAY: ':99.0'
     steps:
     - uses: actions/checkout@v4
     - name: Set up Python ${{ matrix.python-version }}
@@ -22,7 +23,8 @@ jobs:
         python-version: ${{ matrix.python-version }}
     - name: Install dependencies
       run: |
-        sudo apt-get install -y libegl1
+        sudo apt-get install -y libegl1 libxkbcommon-x11-0 libxcb-icccm4 libxcb-image0 libxcb-keysyms1 libxcb-randr0 libxcb-render-util0 libxcb-xinerama0 libxcb-xfixes0 x11-utils
+        /sbin/start-stop-daemon --start --quiet --pidfile /tmp/custom_xvfb_99.pid --make-pidfile --background --exec /usr/bin/Xvfb -- :99 -screen 0 1920x1200x24 -ac +extension GLX
         python -m pip install --upgrade pip
         pip install tox tox-gh-actions
     - name: Test with tox