Added signup-form e2e tests to CI (#16860)

refs https://github.com/TryGhost/Team/issues/3298

This adds a new GitHub workflow that runs the signup-form E2E tests,
only when files in `ghost/signup-form` have changed

.github/workflows/signup-form-tests.yml

@@ -0,0 +1,51 @@
+name: Signup Form Tests
+on:
+  pull_request:
+    paths:
+      - 'ghost/signup-form/**'
+  push:
+    branches:
+      - main
+      - 'v5.*'
+    paths:
+      - 'ghost/signup-form/**'
+env:
+  FORCE_COLOR: 1
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
+jobs:
+  e2e:
+    runs-on: ubuntu-latest
+    if: github.event_name == 'push' || (github.event_name == 'pull_request' && !startsWith(github.head_ref, 'renovate/'))
+    name: E2E
+    env:
+      CI: true
+    steps:
+      - uses: actions/checkout@v3
+      - uses: actions/setup-node@v3
+        with:
+          node-version: "16.13.0"
+          cache: yarn
+
+      - run: yarn --prefer-offline
+
+      - name: Install Playwright
+        run: npx playwright install --with-deps
+
+      - run: yarn workspace @tryghost/signup-form run test:e2e
+
+      - name: Upload test results
+        if: always()
+        uses: actions/upload-artifact@v3
+        with:
+          name: playwright-report
+          path: playwright-report
+          retention-days: 30
+
+      - uses: tryghost/actions/actions/slack-build@main
+        if: failure() && github.event_name == 'push' && github.ref == 'refs/heads/main'
+        with:
+          status: ${{ job.status }}
+        env:
+          SLACK_WEBHOOK_URL: ${{ secrets.SLACK_WEBHOOK_URL }}