Fix difftest.html to work with requirejs
Fix difftest.html to work with requirejs

Test Plan:
Open difftest.html

Auditors: jack

chirilo/perseus,junyiacademy/perseus,sachgits/perseus,chirilo/perseus,ariabuckles/perseus,learningequality/perseus,iamchenxin/perseus,alexristich/perseus,daukantas/perseus,daukantas/perseus,ariabuckles/perseus,junyiacademy/perseus,ariabuckles/perseus,huangins/perseus,ariabuckles/perseus,abe4mvp/perseus,learningequality/perseus,huangins/perseus,abe4mvp/perseus,iamchenxin/perseus,alexristich/perseus,sachgits/perseus