Bug 993712 - Remove hand-rolled timeout handler from customizableui/test/head.js. r=test-only

I'm trying to smoke out an intermittent orange in bug 993712, and I'm starting to run dry on
ideas. I'm having no luck reproducing the bug locally. What I'm trying to do here is cause
the orange (when it rears its head) to force a mochitest timeout, which will take a screenshot.
I'm hoping that screenshot gives me some clues about what's going on.