move jupyter_kernel_test to a separate job

Author: blink1073

.github/workflows/downstream.yml

@@ -52,7 +52,15 @@ jobs:
         with:
           package_name: jupyter_server
 
-      # Test using jupyter_kernel_test
+  jupyter_kernel_test:
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+
+      - name: Base Setup
+        uses: jupyterlab/maintainer-tools/.github/actions/base-setup@v1
 
       - name: Setup conda ${{ matrix.python-version }}
         uses: conda-incubator/setup-miniconda@v2