Fixing perforce driver/downloader bugs

- Updated/refactored Perforce tests to inject IOInterface dependency.  
- Fixed a bug with Perforce hanging on deleting workspaces.  
- Fixed a bug with Perforce not auto-deleting workspaces correctly.  
- Fixed a bug with Perforce not returning a correct source reference for dev-master packages.  
- Fixed a bug with Perforce not auto-updating dev-master dependencies.