message: sample-code backend test assets are in a dir called backend_test_assets
diff: "scripts": {
  "buildbackend": "cd ./backend && tsc --outDir ../backendlib",
  "cleanbackend": "rimraf lib backendlib",
  - "copybackendassets": "cpx ./backend/assets/**/* ./backendlib/assets",
  + "copybackendassets": "cpx ./backend_test_assets/**/* ./backendlib/assets",
  "pretestbackend": "npm run copybackendassets && rush build",
  "testbackend": "npm run pretestbackend && node ../../node_modules/@bentley/bentleyjs-tools/scripts/test --packageRoot=\"../../\" --testDir=\"./backendlib\"",
  "build": "npm run buildbackend",
  