Trying to write tests for a new implementation of FSFile it's difficult to figure out what the expected behaviour should be for various inputs.

e.g.:

• If fileOffset < 0, should it error? With what exception?
• If fileOffset + dest.remaining() >= getLength(), should it read the data it can, and leave the buffer position at the end, or should it error?
• If fileOffset >= getLength(), should it just do nothing, or error? With what exception?
• If one of the above is true, but dest.remaining() == 0, should it succeed?
• Are there other edge cases I haven't thought of?