Hackebrot convert test config

Convert `TestGetConfig`, `TestGetConfigWithDefaults` and `TestGetUserConfig` to `py.test` using fixtures for the existing setup/teardown code.

While implementing `back_up_rc` I noticed that the former `unittest.TestCase` does not remove any `~/.cookiecutterrc` that the tests create. I did not change this behaviour in the re-implementation. Please let me know if I shall add this clean-up.

Cheers! :bow: