The X Test Suite has been around for many years to help in the X Server testing process, but the code-base to XTS is crusty and ancient for stressing the X11 protocol. XTS isn't widely used and beyond that there isn't too much continuous integration and recurring tests that go on of the X.Org Server. (This isn't about X.Org Server performance but more functionality testing and ensuring various input bugs, etc don't reappear.)
Hutterer has been working on a new test suite for the X.Org Server code that is based upon googletest and xorg-test for automating input driver testing, bug replication, and general server behavior.
Beyond the test suite itself, he's also calling for developers when they fix an X.Org Server bug to also create a test case to illustrate the issue. This will allow the selection of test cases to expand and ensure that the bug doesn't come back up in the future. Keith Packard has expressed interest in running this collection of tests whenever making a new push to the xorg-server code-base. Right now this work is all focused for testing X input and not any X output/graphics tests.
Additional information on this X integration test suite can be found via Peter's blog. Embedded below is the video of Peter's X testing talk.