test_x11_remote: Fix finding xterm on Mac OS X
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/

martynjarvis/stb-tester,martynjarvis/stb-tester,wmanley/stb-tester,LewisHaley/stb-tester,wmanley/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester,martynjarvis/stb-tester,wmanley/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,wmanley/stb-tester,wmanley/stb-tester,martynjarvis/stb-tester