"Night testing" is quite vague and without any further info it's difficult to make any guesses as to when the trains will enter service. Is this unit-specific testing or class-wide testing?
It probably varies. Some of the testing will be class based and involve trains coming in on a platform and changing ends, then returning to do another, then perhaps going into a sidings and back out.. presumably to check every signal and stop board and look for other possible hazards.
Unit testing is to rack up mileage before a train can enter service but given there are already more trains ready than needed for the initial diagrams, I assume that's not having any impact whatsoever.