Added noshadows tests to GitHub actions.

.github/workflows/tests.yml

@@ -0,0 +1,26 @@
+name: Tests
+
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read
+
+jobs:
+  noshadows:
+    runs-on: ubuntu-20.04
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+      - uses: actions/setup-python@v5
+        with:
+          python-version: '3.6'
+      - run: pip install tinycss2
+      - run: python noshadows.py --tests