message: testbed now depends on backed v64
diff: "dependencies": {
  "@bentley/bentleyjs-core": "^7.3.2",
  "@bentley/geometry-core": "^5.0.6",
  - "@bentley/imodeljs-backend": "^0.63.1",
  + "@bentley/imodeljs-backend": "^0.64.0",
  "@bentley/imodeljs-clients": "^4.0.0",
  "@bentley/imodeljs-common": "^0.62.0",
  "@bentley/imodeljs-electronaddon": "~10.0.0",
  