Download and use 16-bit SkiFree for NE tests

[disinvite]

Resolves #3. We sort of landed on using existing binaries to test the binary image processing rather than concocting examples ourselves, at least for now.

SkiFree needs no introduction and the EXE is small. To test locally you can get the 16-bit version from Chris Pirih's site, from archive.org, or some old floppy disk you have on hand.

This fixes a bug with NE header unpacking and adds some rudimentary tests. I didn't expand the functionality yet because I'm not sure how we should handle 16-bit addresses.

We cache the download but the cache is not shared between branches so maybe using archive.org is not the best thing long-term. As we add more sample binaries we may want to parameterize the CI task and the way we load the files using pytest. Is that something we should figure out now or later?

[jonschz]

Looks good! Only one small detail: Maybe we want to add some canonical directory to the .gitignore if one wants to keep the files there locally for testing? We could also change the CI scripts to download them there. I'd still keep the option to specify the files by path if one wants to keep them separate.

[disinvite]

Good idea! I'll look into that and see if we can add it here. It would make adding more sample binaries easier.

Should that directory just be under /tests? We could maybe try to autoload it unless you specify an alternate location at the command line.

[jonschz]

How about tests/binaries or something along these lines? Looking there by default sounds like a great idea, it'll simplify IDE integration. Anything will do as far as I am concerned, no need to wait for another approval.