Add tests for states.svn. Ensure svn.latest includes changes in return

This pull request addresses issue #4064. It includes changes in the dictionary returned from `svn.latest`, essentially mirroring the format of changes returned by `git.latest`. 

A few notes:

* There were no integration tests for `states.svn`, so I wrote a test class for it, again largely mimicking the structure of the integration tests for `git.latest`.
* For the test SVN repository, I somewhat arbitrarily chose the test subfolder of the Apache Foundation's httpd library. I figured this has a good chance of sticking around at that location  over the long haul.

This is my first contribution to this project, so let me know if I did anything wrong or you need anything else etc.

I'm working on a deployment against an active SVN repository right now, so I'd really like to have proper changes behavior in `svn.latest`!