Add OpusCustom Test Script

Created test_opus_custom script which does a more heavy evaluation of the various use cases of OpusCustom, testing:
- Mixed float/fixed use cases
- Mixed Opus/OpusCustom use cases
- Wide mixture of run-time configurables
- RMS difference (if RESYNTH) is defined

Signed-off-by: Jean-Marc Valin <jeanmarcv@google.com>