Bug 1033481 - Use a .mozbuild file rather than a .mk in m/a/tests/background/junit3.

This is the android-sync side of the equation.  The idea is to unify the
"out-of-tree project management tools" used by android-sync,
MozStumbler, and the Search Activity.
Bug 1033481 - Use a .mozbuild file rather than a .mk in m/a/tests/background/junit3. r=rnewman