Fix flaky stratifiedImputation test

[egillax]

This test fails occasionally:

── Failure ('test-featureEngineering.R:279:3'): createStratifiedImputationSettings correct class ──
nrow(stratifiedMeans) == 8 is not TRUE

`actual`:   FALSE
`expected`: TRUE 

[egillax]

This one is bad as well:

── Error ('test-demographicSummary.R:63:3'): getDemographicSummary ─────────────
Error in ``$<-.data.frame(tmp`, "PersonCountWithOutcome", value = numeric(0))`: replacement has 0 rows, data has 22
Backtrace:
▆

1. └─PatientLevelPrediction:::getDemographicSummary_survival(...) at test-demographicSummary.R:63:3
2. ├─base::$<-(*tmp*, "PersonCountWithOutcome", value = <dbl>)
3. └─base::$<-.data.frame(*tmp*, "PersonCountWithOutcome", value = <dbl>)

[egillax]

Fixed in #510