Get retry test instructions off-by-one

It looks like the returned unused instructions are off-by-one.

For example take the following:
- Create a new retry test X with instructions:
`map[storage.objects.insert:[return-503-after-257K return-503 return-503 return-503 return-503]`
- Upload the first 256k chunk, which returns a 200. No test instructions should be used up here.
- Try uploading the next 256k chunk twice. Get 2 503s.
* 2 instructions should be consumed (`return-503-after-257K` and `return-503`)
- Get the retry test and inspect the instructions:
`map[storage.objects.insert:[return-503 return-503 return-503 return-503]]`

Expected: `map[storage.objects.insert:[return-503 return-503 return-503]]`

I inserted a delay of 30 seconds and saw the same thing.

Note: this is not an issue with http bucket lists.