Send ArrayBuffer, not Uint8Array in tests

This matches the true contents of a WebSocket 'message' event, so should
be a more realistic test.

tests/fake.websocket.js

@@ -58,8 +58,8 @@ export default class FakeWebSocket {
         // 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.subarray(i, i+1);
-            this.onmessage(new MessageEvent("message", { 'data': buf }));
+            let buf = data.slice(i, i+1);
+            this.onmessage(new MessageEvent("message", { 'data': buf.buffer }));
         }
     }
 }