Update tests.yaml

Since gmsh depends on OpenGL utilities, one needs to install them on the Ubuntu runner before running tests.

Author: livoire13

.github/workflows/tests.yaml

@@ -25,6 +25,9 @@ jobs:
         with:
           python-version: ${{ matrix.python-version }}
 
+      - name: Install system dependencies
+        run: sudo apt-get update && sudo apt-get install -y libglu1-mesa
+
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip