test: verify that singleton continuous variables are put into the dataset drawer

[seve]

Related feature: #615

We may want to also verify categorical singletons are put into the dataset drawer as well

[tihuan]

Good call thank you! Ideally e2e test should be part of the implementation unless there's a rush to get the implementation in without e2e test in place!

[niknak33]

Nik to Sync with Ronen on this ticket.