Add GitHub action to run tests for PRs

Author: whimboo

.github/workflows/test.yml

@@ -0,0 +1,45 @@
+name: Tests
+
+on:
+  pull_request:
+    branches:
+      - master
+  push:
+    branches:
+      - master
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Set up Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: '20'
+          cache: 'npm'
+
+      - name: Set up Firefox Nightly
+        uses: browser-actions/setup-firefox@v1
+        with:
+          firefox-version: latest-nightly
+
+      - name: Verify Firefox Nightly installation
+        run: firefox --version
+
+      - name: Install dependencies
+        run: npm ci
+
+      - name: Run tests
+        run: FIREFOX_BINARY=firefox npm test
+
+      - name: Upload screenshots on failure
+        if: failure()
+        uses: actions/upload-artifact@v4
+        with:
+          name: test-screenshots
+          path: test/screenshots/
+          if-no-files-found: ignore