Clearly towards the tail-end of last season they were overloading new aero pieces on the car, which suggests the core chassis was likely to remain similar to 2014, but to be honest the chassis itself changes little year on year (remember that up until recently (last 4 year or so?) Force India were generally accepted to be running a modified Jordan (yes, Jordan) chassis), which had gone through several major rule and safety changes.
It's what you bolt onto the chassis which matters most, and that's where McLaren have fallen short in recent times, and why Peter Prodromou has been brought in to lead the aero department.
Passing the test suggests the crash structures and basic cooling systems are established and they're happy with what they've presented, but given they were willing to run a development Honda plant at Silverstone and Abu Dhabi, clearly they've known the basic numbers for some time. It's not a major coup, but it's better to have passed those tests early.