I noticed we were really undertesting all of the things that handle
paths and file names. I gave this some love and a little clean up where
we weren't doing the right thing in RazorSourceDocument.
Also changed the template engine tests to use the
FileSystemRazorProject. These tests are already using the files on disk
as inputs. I turned off checksums for these since they now have the full
file path, and that would not be portable.
