Comment 1 for bug 1957910

That's because of we remove filename extension in this commit.
https://git.launchpad.net/checkbox-support/commit/?id=d1a4fd6788291a502b4e62bd708134a3e45e09fa

The fwts would attached ".log" automatically. So if we provided "fwts", then fwts would actually store output to "fwts.log"