Improve tests for the spice level 2 reader

[nabobalis]

Provide a general description of the issue or problem.

We recently closed some bugs with the spice reader in sunraster.

https://github.com/sunpy/sunraster/issues/196 and https://github.com/sunpy/sunraster/issues/202

Issue is, we don't have any tests for these cases, partly due to the size of the files, downloading them for a test would take too long and we can not add them to the repository.

We would need a way to create these files based on nothing but the header/metadata for each file and using random data to ensure the reader works.

[ebuchlin]

Same for #208. I'm thinking of a way to create these test files.