Where is the Microsoft.VisualStudio.TestTools.UnitTesting namespace on VS2010?

It’s called Microsoft.VisualStudio.QualityTools.UnitTestFramework.dll.

You can find it in C:\Program Files (x86)\Microsoft Visual Studio 10.0\Common7\IDE\PublicAssemblies\Microsoft.VisualStudio.QualityTools.UnitTestFramework.dll.