Bug 230610

r282082 changed the way xvfb is spawned by providing a PIPE for stdout and stderr instead of redirecting to /dev/null. The problem is that stderr is only read in case of execution failure, so when nothing fails we end up filling the buffer that is never read. We can try to find a better solution to show the stderr output in error messages, but for now we can revert that part to redirect stderr to /dev/null to make the tests work again.