+str dsl consistency spec also investigates return types now

Slightly improves `DSLFactoriesConsistencySpec` in the spirit of #16433 - by also looking at the declared return type.

Things can still be wrong - for example if javadsl declared `: Source[S]` but inside it returns `new KeyedSource` - @bantonsson found such a mistake recently. Still, it's another improvement to at least on the signature level make sure those are matching.