It can't work, it should be `app.cfg` instead of `self.cfg`.

Thing is, this function is only used by the regtester, because it wants control over each individual step being run, i.e. it doesn't use `run_all_steps`.

I know, totally wrong, and there's an issue to fix this somehow. I still don't know how though, and I won't spend time on it now.

I'll retest the FFTW build with `--regtest --sequential`, which will trigger this block of code.