Introduce JS utility function named "getTimeseriesCollForDDL(origColl)" to make easier writing timeseries test that works both for viewless tiemeseries collection and legacy timeseries collection.

The behavior of this function will depend on the CreateViewlessTimeseriesCollections feature flag:
 - When the feature flag is enabled, this function will be a no-op and will return the main collection.

• When the feature flag is disabled, this function will return the underlying system.buckets collection.