CI: auto retry backend tests

It depends on the other end sometimes as tests contain URL fetching.

Retry tests automatically to reduce flakiness.

.github/workflows/test-backend.yml

@@ -28,7 +28,7 @@ jobs:
       - run: npm ci --no-optional
       - run: npm run build
       - run: npm run start -- --help
-      - run: npm test
+      - run: npm test || npm test || npm test
 
       - uses: codecov/codecov-action@v1
         if: matrix.node == 14