Refactor unit tests with large arrays - move to .npy files in the repo

[coreylowman]

Unit tests should be easy to scan. large arrays make this almost impossible and add a lot of bloat.

Originally posted by @coreylowman in https://github.com/coreylowman/dfdx/pull/120#discussion_r929261072

[coreylowman]

Downside of this is that it becomes unclear what is in the npy files & how they are produced. Ofc it is already unclear how the arrays are produced, so this is a bigger issue.