Make WebAudio test work in debug mode.

- Add a dummy FFTFrame implementation.
- The audio loop starts ASAP but the data will be
  output only after the HRTF database is loaded.
  Thus the need to check for the ramp pattern anywhere
  in the data output.
- Search for HRTF samples in the uninstalled directory
  as fallback.

Reviewed by: Tullio Lucena