Add tests to ci

Test pyflix and simulation using pytest.
Fix running the sim in non-tty environment.
Test presence of some essential files in the docs.

.github/workflows/build.yml

@@ -84,15 +84,16 @@ jobs:
       run: sudo apt-get install -y libsdl2-dev
     - name: Build simulator
       run: make build_simulator
-    - name: Run simulator
+    - name: Install Python requirements
+      run: sudo apt install -y python3-pip && pip3 install -r tools/requirements.txt && pip3 install pytest
+    - name: Run simulator and tests
       env:
         GAZEBO_MODEL_PATH: ${{ github.workspace }}/gazebo/models
         GAZEBO_PLUGIN_PATH: ${{ github.workspace }}/gazebo/build
+        GAZEBO_MODEL_DATABASE_URI: '' # disable downloading models
       run: |
-        OUT=$(timeout -k 10s 120s gzserver --verbose gazebo/flix.world 2>&1 | tee /dev/stderr)
-        if echo "$OUT" | grep -Pq "\[Err\](?! \[RenderEngine)"; then
-          exit 1
-        fi
+        gzserver --verbose gazebo/flix.world &
+        pytest --capture=no --verbose tools/test.py
     - uses: actions/upload-artifact@v7
       with:
         name: gazebo-plugin-binary