node tests: Make with_overrides() more granular.

The with_overrides() function no longer works at the module
level, so you can temporarily override one function in a module
without breaking more permanent monkey patches.

This makes us slightly more vulnerable to unintentional test
leakages, but it solves the problem where you often need lots
of temporary overrides for writer functions but you want to
keep a more permanent override for some sort of reader function.

frontend_tests/zjsunit/namespace.js

@@ -61,15 +61,19 @@ exports.with_overrides = function (test_function) {
         var parts = name.split('.');
         var module = parts[0];
         var func_name = parts[1];
-        var module_impl = {};
-        module_impl[func_name] = f;
-        set_global(module, module_impl);
+
+        if (!_.has(global, module)) {
+            set_global(module, {});
+        }
+
+        global[module][func_name] = f;
 
         clobber_callbacks.push(function () {
             // If you get a failure from this, you probably just
             // need to have your test do its own overrides and
             // not cherry-pick off of the prior test's setup.
-            set_global(module, 'UNCLEAN MODULE FROM PRIOR TEST');
+            global[module][func_name] =
+                'ATTEMPTED TO REUSE OVERRIDDEN VALUE FROM PRIOR TEST';
         });
     };