Does Visual C# 2010 Express have a unit testing feature?
Nothing built in, but you can always use nUnit with it.
MSTest comes bundled with the Pro edition and above.
Visual Studio Express editions have the limitation that plugins/addins are expressely disallowed. They do not ship with a built-in testing solution, and you cannot add-in one.
Your best/only option it to use a standalone test runner, such as nUnit, mspec, etc... and run it externally from VSE.
As has been stated, the Express versions do not have any built-in, and do not allow add-ins for, this functionality, but you can use an external tool, e.g. NUnit.
You can also set up a command to run from the 'Tools->External Tools' menu option from within Visual Studio Express and run your test runner from that if you wish.
Here is a link that shows how to do it with VS C# 2008 Express, (scroll down to the end) but I think it should hold true for 2010 as well.