They obviously have their reasons for using an interim car for the first test, and it obviously doesn't impact them that much not using the new car otherwise they would of pushed the launch date forward.
When you think about it, the cars won't change much from last year in relative terms, so blocking off the diffuser and fitting a moveable rear wing isn't too hard, fitting KERS however may be difficult though.
This to me means that Mclaren have decided to stick with a push rod setup, other wise they would be running a new car with pullrod because the differences are very big.
Just my thoughts