Adding registry node load tests

This is my attempt at adding registry init/load specific unit tests. There's more to be added later but I think what I've got so far is ready to be merged upstream.

I understand that the usage of the should module is encouraged but I decided to call assert as it's more straightforward to just simply fail a state with it by calling assert.fail. With should, I would need to craft a fake failing statement which in my opinion is harder to read.
Don't want to make `defaultNodesDir` customisable, if the only reason to do so is for the tests.

For now, I'd prefer the `load` function take an optional argument that overrides the default location if set.