Run Playwright tests in CI

So that we have some test coverage in this repo.

.github/workflows/test.yml

@@ -0,0 +1,39 @@
+name: Run tests
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    environment: Happo secrets
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: '20'
+          cache: 'yarn'
+
+      - name: Install dependencies
+        run: yarn install --frozen-lockfile
+
+      - name: Install Playwright browsers
+        run: npx playwright install chromium
+
+      - name: Start dev server
+        run: |
+          yarn dev &
+          # Wait for server to be ready
+          sleep 5
+
+      - name: Echo key
+        run: echo ${{ secrets.HAPPO_API_KEY }}
+
+      - name: Run tests
+        run: yarn test