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.
Actually no, if I move it in the coming PR it just adds extra noise that it doesn't need (going to be coming shortly).  Essentially every test that uses the MvcRazorHost could potentially use the TestFileSystem was the initial thought.  Will keep this in for now to make coming PR's easier to read.