Stream TestNodesExchangeCorrectBinIndexes flay on Travis

Test network/stream/v2.TestNodesExchangeCorrectBinIndexes is flaky on Travis, but locally is stable, even with race detector.

```
--- FAIL: TestNodesExchangeCorrectBinIndexes (6.48s)
    cursors_test.go:98: bin indexes not equal. bin 2, got 135, want 131
```

https://travis-ci.org/ethersphere/swarm/jobs/585475387