test: Fix unit mock issue in remove-obsolete-dep

We now test extraneousness which means recursively walking `requiredBy`
which won't work if one of them is not an object.

test/tap/unit-deps-removeObsoleteDep.js

@@ -33,7 +33,7 @@ test('removeObsoleteDep', function (t) {
   removeObsoleteDep(test2)
   t.is(child2.removed, true, 'required by no other modules, removing')
 
-  var child3 = {requiredBy: ['NOTEMPTY']}
+  var child3 = {requiredBy: [{isTop: true}]}
   var test3 = {
     requires: [ child3 ]
   }