message: Add `jest-mock` to dependencies
  It's being used by the chat export test, but is not listed as a dependency.
  If we rely on it, we should rely on it.
diff: "jest-canvas-mock": "^2.3.0",
  "jest-environment-jsdom": "^27.0.6",
  "jest-fetch-mock": "^3.0.3",
  + "jest-mock": "^27.5.1",
  "jest-raw-loader": "^1.0.1",
  "matrix-mock-request": "^1.2.3",
  "matrix-react-test-utils": "^0.2.3",
  