node_modules/karma/node_modules/log4js/test/log4js.json

{
  "appenders": [
    { 
      "category": "tests", 
      "type": "file", 
      "filename": "tmp-tests.log", 
      "layout": { 
        "type": "messagePassThrough" 
      } 
    }
  ],
  
  "levels": {
    "tests":  "WARN"
  }
}