Bare bones setup to debug tests

Add $NODE_DEBUG_OPTION to script you want to debug, so package.json has a following section:

{
  "scripts": {
    "test": "mocha $NODE_DEBUG_OPTION"
  }
}

1. Open Run > Edit Configurations... dialog.
2. Add New Configuration (Alt+Insert), a green plus button at top-left.
3. Select npm.
4. Name is whatever, e.g. test.
5. For Scripts select test.
6. Click OK.
7. Debug this configuration using a bug icon at top-right corner of the main window.