test: reduce flakiness in test-heapdump-http2

By the time the response event is emitted on the client's side, the
file may have already been fully piped and the stream pipe
may have been destroyed, so the test should not look for the
stream pipe in the snapshot.

Author: joyeecheung

test/pummel/test-heapdump-http2.js

@@ -48,10 +48,9 @@ server.listen(0, () => {
       { node_name: 'TCP', edge_name: 'native_to_javascript' },
     ]);
 
-    // `Node / StreamPipe` (C++) -> StreamPipe (JS)
-    validateByRetainingPathFromNodes(nodes, 'Node / StreamPipe', [
-      { node_name: 'StreamPipe', edge_name: 'native_to_javascript' },
-    ]);
+    // We don't necessarily have Node / StreamPipe here because by the time the
+    // response event is emitted, the file may have already been fully piped here
+    // and the stream pipe may have been destroyed.
 
     // `Node / Http2Session` (C++) -> Http2Session (JS)
     const sessions = validateByRetainingPathFromNodes(nodes, 'Node / Http2Session', []);