Fix wrong variable name in requiremock's getDefine

The getDefine method check's for existence of the 'arg' variable,
which was renamed to 'name', causing the typeof arg === "undefined"
to always return true

refs #4308

test/js/testlib/requiremock.js

@@ -115,7 +115,7 @@ module.exports = {
     purgeDependencies: purgeDependencies,
     registerDependencies: registerDependencies,
     getDefine: function(name) {
-        if (typeof arg === "undefined") {
+        if (typeof name === "undefined") {
             return registeredDependencies.__define__;
         } else {
             return registeredDependencies[name];