Fixes to Game startup when using a unit test runner.
The Content.RootDirectory cannot be absolute.
Removed local directory reference.
Using AppDomain.CurrentDomain.BaseDirectory seems to be a more reliable way of getting the working dir.
I was wrong.... Content.RootDirectory can have an absolute path.  MonoGame needs fixing here.
Game takes care of disposing GraphicsDeviceManager.
Merge pull request #1530 from SickheadGames/testfix

Unit Test Fixes