Clean up test shutdown

Make sure that the integration tests actually kill off all of their timers, so
that jasmine exits cleanly.

This probably also fixes https://github.com/vector-im/vector-web/issues/1365.

lib/sync.js

@@ -419,6 +419,10 @@ SyncApi.prototype.stop = function() {
     }
     this._running = false;
     if (this._currentSyncRequest) { this._currentSyncRequest.abort(); }
+    if (this._keepAliveTimer) {
+        clearTimeout(this._keepAliveTimer);
+        this._keepAliveTimer = null;
+    }
 };
 
 /**