Fix Test constructor so that it doesn't accidentally mutate global scope

As used by Test.skip itself circa lib/test.js:464

Author: Joshua T Corbin

lib/test.js

@@ -36,6 +36,10 @@ var getTestArgs = function (name_, opts_, cb_) {
 };
 
 function Test (name_, opts_, cb_) {
+    if (! (this instanceof Test)) {
+        return new Test(name_, opts_, cb_);
+    }
+
     var args = getTestArgs(name_, opts_, cb_);
 
     this.readable = true;