You are right, this is incorrect. [TestGameBase.cs:93](https://github.com/mono/MonoGame/blob/develop/Test/Framework/TestGameBase.cs#L93) does set RootDirectory to an absolute path:
```
Content.RootDirectory = AppDomain.CurrentDomain.BaseDirectory;
```
and this works on XNA but fails on MonoGame. I have tested on Windows, WindowsGL, Linux, Mac on a clean clone of develop. Setting this to String.Empty makes the test runner work, so something appears to be broken in MonoGame.

At this point, I just need a way to run the unit tests on my systems, so setting RootDirectory to empty works for me. I will log a bug and leave this to someone who knows how the ContentManager is supposed to work.