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.
Since it's a common assembly it defines the Moq and Xunit.KRunner dependencies.  Better to have the project.json's utilize fewer dependencies imo :smile: