go/roothash/tester: fix flaky RoundTimeoutWithEpochTransition

[ptrus]

Extracted from: #3212

[kostko]

Thanks!

[codecov]

Codecov Report

Merging #3214 into master will increase coverage by 0.10%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3214      +/-   ##
==========================================
+ Coverage   67.58%   67.69%   +0.10%     
==========================================
  Files         370      370              
  Lines       36098    36098              
==========================================
+ Hits        24398    24436      +38     
- Misses       8541     8550       +9     
+ Partials     3159     3112      -47     

Impacted Files	Coverage Δ
go/roothash/tests/tester.go	89.72% <100.00%> (ø)
go/consensus/tendermint/api/errors.go	86.66% <0.00%> (-13.34%)	⬇️
go/consensus/tendermint/apps/scheduler/query.go	71.05% <0.00%> (-7.90%)	⬇️
go/registry/api/grpc.go	36.08% <0.00%> (-7.34%)	⬇️
go/consensus/tendermint/apps/staking/query.go	49.09% <0.00%> (-7.28%)	⬇️
go/worker/storage/service_external.go	51.47% <0.00%> (-5.89%)	⬇️
go/staking/api/grpc.go	54.26% <0.00%> (-5.04%)	⬇️
go/worker/keymanager/handler.go	60.37% <0.00%> (-3.78%)	⬇️
go/consensus/tendermint/apps/staking/state/gas.go	81.03% <0.00%> (-3.45%)	⬇️
go/consensus/tendermint/staking/staking.go	57.45% <0.00%> (-3.32%)	⬇️
... and 34 more

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