JSON files being written to app directory after running reference

Hi there,

I'm using backstop in a js file rather than through the CLI and I get random json files being added after the reference is run. This is using chrome as the engine. Example of the json file below

```
{
"compareConfig": {
"testPairs": []
}
}
```

I tracked down the source of this file to the function `writeCompareConfigFile` in the file `core/util/createBitmaps.js` So I'm wondering what this is used for? I can get a docker example setup if needed, so you can reproduce this