Add support for 20KBP filter

In the Audio SWG call on 2023-04-28, it was decided to use the 20KBP-filter (from ITU filter executable) for all BS.1534 tests and the mixed/muisc P.SUPPL800 tests. This needs to be added to the scripts.

20KBP   20-20k Hz Flat bandpass FIR filter w/ factor 1:1 at sf=48kHz