test workflow: clone submodule

Author: Gerit Wagner

.github/workflows/tests.yml

@@ -13,6 +13,9 @@ jobs:
     runs-on: ${{ matrix.platform }}
     steps:
       - uses: actions/checkout@v4
+        with:
+          submodules: recursive
+          fetch-depth: 0
 
       - name: Set up Python ${{ matrix.python-version }}
         uses: actions/setup-python@v4