Additional/Modified FATE tests improve code coverage from 63.7% to 98.1%.

Changed fate-suite sample files:
* filter/hdcd-mix.flac (958K) added. It is a much better test than
  filter/hdcd.flac (910K), which is now unused, but can't be removed.
* filter/hdcd-fake20bit.flac (168K) added. It is the first second of
  filter/hdcd.flac, with the 16-bit LSB copied into bit 20 of a 24-bit
  stream. There isn't an actual non-16-bit HDCD sample available to test.
Signed-off-by: Burt P <pburt0@gmail.com>