updates-testing repo for CentOS 6?

Solution 1:

Available Repositories for CentOS

Read under 'Additional CentOS Provided Repositories'. I believe the repository you are looking for is CentOS-Testing. You'll have to install it manually, as it doesn't come installed by default. The CentOS-Testing repo can be found at:

http://dev.centos.org/centos/6/testing/CentOS-Testing.repo