Add upgrade test

.github/workflows/upgrade-test.yaml

@@ -0,0 +1,41 @@
+on:
+  pull_request:
+  push:
+  workflow_dispatch:
+
+jobs:
+  upgrade-test:
+    if: ${{ github.ref != 'refs/heads/master' }}
+    runs-on: ubuntu-18.04
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-python@v2
+        with:
+          python-version: 3.6
+      - name: Set BOOTSTRAP_PIP_SPEC value
+        run: |
+          echo "BOOTSTRAP_PIP_SPEC=git+https://github.com/$GITHUB_ACTOR/the-littlest-jupyterhub.git@$GITHUB_SHA" >> $GITHUB_ENV
+      - name: Build systemd image
+        run: |
+          .github/integration-test.py build-image
+      - name: Run basic tests
+        run: |
+          .github/integration-test.py run-test \
+            --bootstrap-pip-spec "$BOOTSTRAP_PIP_SPEC" \
+            basic-tests test_hub.py test_proxy.py \
+            test_install.py test_extensions.py \
+            --upgrade
+      - name: Run admin tests
+        run: |
+          .github/integration-test.py run-test \
+            --installer-args "--admin admin:admin" \
+            --bootstrap-pip-spec "$BOOTSTRAP_PIP_SPEC" \
+            basic-tests test_admin_installer.py \
+            --upgrade
+      - name: Run plugin tests
+        run: |
+          .github/integration-test.py run-test \
+            --bootstrap-pip-spec "$BOOTSTRAP_PIP_SPEC" \
+            --installer-args "--plugin /srv/src/integration-tests/plugins/simplest" \
+            plugins test_simplest_plugin.py \
+            --upgrade