UT: random failure of TestSnapSyncWithBlobs

this case failed randomly in github CI, the ratio of around 20%
but can not be reproduced locally
  go test -p 1 -v -run TestSnapSyncWithBlobs -count 100
Add extra wait time for snapsync to have a try...

eth/sync_test.go

@@ -167,6 +167,10 @@ func testChainSyncWithBlobs(t *testing.T, mode downloader.SyncMode, preCancunBlk
 	})
 	// Wait a bit for the above handlers to start
 	time.Sleep(250 * time.Millisecond)
+	if downloader.SnapSync == mode {
+		// Wait extra 250ms for snap sync, as snap sync will take more round of p2p message
+		time.Sleep(250 * time.Millisecond)
+	}
 
 	// Check that snap sync was disabled
 	op := peerToSyncOp(mode, empty.handler.peers.peerWithHighestTD())