Flaky test: test/e2e-go/features/participation TestPartkeyOnlyRewards

## Description
Travis Test Errors Reference Spreadsheet for details on cases where this test has failed: https://docs.google.com/spreadsheets/d/1apjaNvZodnTSIdZaQlpMaqf0nzbKgSYdIWHqvmINHS8/edit#gid=0

### Example Failure
Time: Wed 7/21/2021
Build: https://app.travis-ci.com/github/algorand/go-algorand/jobs/525817892#L4550
Travis Test: Ubuntu AMD64 Integration Test
Package: test/e2e-go/features/participation
Test: TestPartkeyOnlyRewards

```
algod(11830) : created by github.com/[secure]/go-[secure]/network.(*WebsocketNetwork).Start
fixture.go:105:
Error Trace: participationRewards_test.go:159
Error: Received unexpected error:
fixture.WaitForRound took 535.19 seconds between round 33 and 34
Test: TestPartkeyOnlyRewards
```

### Acceptance criteria:
- Reproduce & resolve the failure