message: added backend-offline command in package.json
diff: "test:bentley": "cd core/bentley && npm run test",
  "test:geometry": "cd core/geometry && npm run test",
  "test:backend": "cd core/backend && npm run test",
  + "test:backend-offline": "cd core/backend && npm run test --offline",
  "test:backendwebpackformobile": "cd core/backend && npm run webpackformobile",
  "test:frontend": "cd testbed && npm run build && node bin/testbed.js",
  "test:sample-code": "cd sample-code && npm run test",
  