Don't split large WebSocket data in tests

It takes too much time and can make the tests fail.

tests/fake.websocket.js

@@ -55,12 +55,16 @@ export default class FakeWebSocket {
     }
 
     _receiveData(data) {
+        if (data.length < 4096) {
             // Break apart the data to expose bugs where we assume data is
             // neatly packaged
             for (let i = 0;i < data.length;i++) {
                 let buf = data.slice(i, i+1);
                 this.onmessage(new MessageEvent("message", { 'data': buf.buffer }));
             }
+        } else {
+            this.onmessage(new MessageEvent("message", { 'data': data.buffer }));
+        }
     }
 }