Add unit tests for extensions

• Add unit tests for opus_packet_extensions_count, opus_packet_extensions_parse, and opus_packet_extensions_generate
• Add various assertions for NULL buffers or negative length arguments
• Add explicit check for invalid length in opus_packet_extensions_count
• Check for extension id >127 in opus_packet_extensions_generate

Open question: Should 0 length buffer argument immediately return OPUS_INVALID_PACKET