Added instance var for data manager singleton for tests.
Added instance var for data manager singleton for tests.

iOSTestApps/xkcd-Open-Source,iOSTestApps/xkcd-Open-Source,IsaacChapman/xkcd-Open-Source,Suninus/xkcd-Open-Source,Suninus/xkcd-Open-Source,IsaacChapman/xkcd-Open-Source,IsaacChapman/xkcd-Open-Source,mamaral/xkcd-Open-Source,mamaral/xkcd-Open-Source,mamaral/xkcd-Open-Source