Fix test: panic when decoded data is wrong size

This started life as a dev utility so I was only printing the message.
Now that it's a `#[test]`, I'll have it panic on the error path.

tests/codec.rs

@@ -62,7 +62,7 @@ fn decode_qoi_samples() -> Result<(), TestError> {
     let data: Vec<u8> = PixelIterExpander::try_new(decoder).unwrap().collect();
     let result = writer.write_image_data(&data);
     if let Err(res) = result {
-        eprintln!("{}", res);
+        panic!("{}", res);
     }
     return Ok(())
 }