Flaky test: TestRewardUnitThreshold

## 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: Fri 6/25/2021
Build: https://travis-ci.com/github/algorand/go-algorand/jobs/518879318#L1950
Travis Test: Ubuntu AMD64 Integration Test
Package: test/e2e-go/features/participation
Test: TestRewardUnitThreshold

```
algod(12456) : === CONT TestPartkeyOnlyRewards
algod(12456) : algod(12456) : Exiting on terminated
algod(12416) : Exiting on terminated
algod(12442) : Exiting on terminated
algod(12472) : Exiting on terminated
fixture.go:121:
Error Trace: participationRewards_test.go:170
Error: Received unexpected error:
fixture.WaitForRound took 477.03 seconds between round 41 and 42
Test: TestPartkeyOnlyRewards
algod(12456) :
algod(12456) :
algod(12456) :
algod(12456) :
algod(12456) : panic: (*logrus.Entry) (0x1b8f160,0xc00034e9a0) [recovered]
algod(12456) : panic: (*logrus.Entry) (0x1b8f160,0xc00034e9a0)
algod(12456) : goroutine 16633 [running]:
algod(12456) : github.com/[secure]/go-[secure]/logging.logger.Panic.func1(0xc0002607e0, 0xc000010568)
algod(12456) : github.com/[secure]/go-[secure]/logging/log.go:252 +0xd5
algod(12456) : panic(0x1b8f160, 0xc00034e9a0)
algod(12456) : runtime/panic.go:975 +0x3e3
algod(12456) : github.com/sirupsen/logrus.Entry.log(0xc000260770, 0xc002ed5bc0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
```
The following happened twice:
```
algod(12055) : panic: (*logrus.Entry) (0x1b8f160,0xc00033da40) [recovered]
algod(12055) : panic: (*logrus.Entry) (0x1b8f160,0xc00033da40)
algod(12055) : goroutine 28820 [running]:
algod(12055) : github.com/[secure]/go-[secure]/logging.logger.Panic.func1(0xc0001407e0, 0xc0000c6558)
algod(12055) : github.com/[secure]/go-[secure]/logging/log.go:252 +0xd5
algod(12055) : panic(0x1b8f160, 0xc00033da40)
algod(12055) : runtime/panic.go:975 +0x3e3
algod(12055) : github.com/sirupsen/logrus.Entry.log(0xc000140770, 0xc0033945a0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
```