Xvfb processes orphaned with each test run

[gsf]

Xvfb processes are left running after tests finish, and subsequent test runs hang until the processes are killed manually. I was able to get some success with a process.on('exit', function () {childProcess.kill()}) in headless, but ugh this Xvfb business is touch and go. This is on Arch Linux running testling 1.7.0.

I believe this issue is also mentioned in #71, though it is not constrained to custom servers.