"Live" unit testing is nothing but automated execution of unit tests that may have impacted by your code change and it provides the real time results.
Currently, I am using NCrunch which provides the same mechanism and it really works well. However Microsoft has directly given that capability without using any extension or any other external tools.
For configuring the Live Testing settings, you need to browse to Tools-->Options and than select "Live Unit Testing".
Real time test case execution is really helpful and if you don't have Visual Studio Enterprise edition than you can still go for NCrunch.
Thanks for reading!!!