Add a regression test for the backing up and overwriting logic.

[kindaro]

As said in the overview of the package:

1. Prettified files are backed up with the extension *.previous.

   1. Files that are already pretty are not backed up.

      This ensures that running cabal-prettify twice in a row does not overwrite the original.

This is hazardous. I should add a regression test.

[kindaro]

I figure this gets checked by the new examples test suite added in #17.

[kindaro]

Actually it does not. It only checks that these files get created with the right content. We need to run cabal-prettify two times to check that the logic is working right.