add cling test

Author: blink1073

.github/workflows/downstream.yml

@@ -45,6 +45,8 @@ jobs:
         with:
           package_name: jupyter_server
 
+      # Test using jupyter_kernel_test
+
       - name: Setup conda ${{ matrix.python-version }}
         uses: conda-incubator/setup-miniconda@v2
         with:
@@ -58,5 +60,6 @@ jobs:
           git clone https://github.com/jupyter/jupyter_kernel_test.git
           cd jupyter_kernel_test
           conda env update --name jupyter_kernel_test --file environment.yml
+          conda install -c conda-forge xeus-cling
           pip install -e ".[test]"
           python -m unittest -v