add a (working) gc test :)

Note: this needs isaacs/tap-runner#6.
With older tap-runner versions, it complains about missing
`gc()` function.

test/gc.js

@@ -0,0 +1,56 @@
+#! node --expose-gc
+var dnode = require('../');
+var test = require('tap').test;
+var weak = require('weak');
+
+test('gc', function (t) {
+    t.plan(2);
+    var port = Math.floor(Math.random() * 40000 + 10000);
+
+    var gcLoopIterations = 0;
+    var server_ok = false, client_ok = false;
+
+    function forced_gc_loop() {
+        gc();
+        if (++gcLoopIterations < 50) {
+            setTimeout(forced_gc_loop, 100);
+        } else {
+            if (!server_ok || !client_ok) t.end();
+            setTimeout(function() {
+                process.exit();
+            }, 1000);
+        }
+    }
+    forced_gc_loop();
+
+    function onSuccess() {
+        t.end();
+        setTimeout(process.exit, 500);
+    }
+
+    var server = dnode({
+        callMeBack : function (fn) {
+            fn('calling back!');
+            weak(fn, function() {
+                t.ok(true);
+                server_ok = true;
+                if (client_ok) onSuccess();
+            });
+        }
+    }).listen(port);
+
+    server.on('ready', function () {
+        dnode.connect(port, function (remote, conn) {
+            function weakCb() {
+                t.ok(true);
+                client_ok = true;
+                if (server_ok) onSuccess();
+            }
+            (function() {
+                var cb = function(){};
+                remote.callMeBack(cb);
+                weak(cb, weakCb);
+            })();
+        });
+    });
+});