test_x11_remote: Tear down Xorg & xterm in event of test failure

Otherwise the next time you run the tests it says "Skipped: There is
already a display server running on :99".
test_x11_remote: Fix finding xterm on Mac OS X

Running this test on OS X requires installing XQuartz[1], which installs
xterm into /opt/X11/bin. (You'll also have to install xdotool using
macports or similar.)

[1]: http://xquartz.macosforge.org/