I actually found more information on this. When using the Nunit for testing, it requires changing the debug startup settings. This gives the object test bench problems:

I had the 0x80004005 error too. The problem is with the project file's Debug settings. Right-click the project file, select "Properties". Select the "Debug" tab, and make sure "Start Action" is set to "Start Project". By selecting this, you should be able to select the "Enable the Visual Studio hosting process" checkbox at the bottom of the form. Not having this checkbox checked is causing the error with Object Test Bench.