Support running tests on Windows

[kurtmckee]

Hi Ned, this fixes two problems that I encountered.

First, it resolves hard-coded '/' directory separators in the unit tests -- on Windows these appear as backslashes, and now the unit tests should be flexible enough to allow for this.

Second, it explicitly specifies the directories for Black to scan. In my environment I set up a virtual environment, and Black spent a long time scanning every Python file in the virtual environment. I've separated the changes in case you don't want this change included.

Please let me know if you have any questions or want something changed or done a different way.

[kurtmckee]

Rebased on master now that #17 is merged.

[nedbat]

Thanks! In the future, I might come up with a more succinct way to deal with the os.path.sep thing, but this is good for now!

[kurtmckee]

Thanks Ned!