Create a lightweight ep-data mock repo for testing

[chrismytton]

Rather than having to test with the full everypolitician-data repo we should create a very small repo that has the same behaviour, but is much quicker to clone. This would make testing changes much simpler.

[chrismytton]

This is less painful as of #71 because we're only doing a shallow clone. But it would still be generally useful to have a way of testing the rebuilder without having to do a full build.