Add CommonTest project - Initially just for Tag Helpers feature branch.

- Added a common test to share common test files.  Moved the TestFileSystem from the razor host project so we could utilize it in other test projects.

#1268

**NOTE:** If you everyone wants this sooner (aka in dev) let me know and I can move it there.
why add this dependency before anything in this project needs a class from CommonTest?
similar question for projects other than Microsoft.AspNet.Mvc.Razor.Host.Test