flac: add test (encode and decode "raw audio")

Here's a test for FLAC. Since the codec is lossless, it's easy to feed it raw data and assert it's the same data after encode+decode.
use

```rb
raw_data = "pseudo audio data that stays the same \x00\xff\xda"
raw_data.force_encoding("ASCII-8BIT") if raw_data.respond_to?(:force_encoding)
```