I know what you are saying and if it was something electrical in the plane’s systems themselves as the cause, then yes that is a problem.
What I am saying is maybe one of the pieces of test equipment started on fire/exploded and that in turn caused plane systems to catch fire.
In any case, we will know soon enough.
At work (and partially at home) I'm surrounded with test equipment. Every unit has a fuse or two, and they will blow at any hint of a problem. I very much doubt that any non-standard test equipment was used, since it's much cheaper to put together a custom data acquisition system from COTS parts rather than to roll your own. I'd guess they have instruments connected through GPIB or LXI to computers, and they are just recording everything, and analyzing it on the ground.
Inside the test aircraft the equipment is likely mounted in open racks. You have to mount it, or else it will slide and squash you (it's heavy) as the aircraft experiences various acceleration. These are open racks, so it would be very obvious where the problem is, especially if there is an operator in front of every rack (judging by the cited 40 people on board.)
And of course if the fire started in a piece of test equipment and then ignited a wall, or a chair, then again it would be very obvious what's happening; perhaps they wouldn't even need to land, as long as they could put it out in place. That landing was paid a heavy price for, PR-wise. But reports leave an impression that the fire was hidden, like in one of cable channels. That is certainly possible; one of the Shuttles developed a very similar problem some years ago.
With regard to knowing the cause, we may or may not know it. Boeing has no particular obligation to tell us anything. Of course if they don't talk they may create an impression that they are hiding things, and that is not a good message to send. We'll wait and see, of course - what else can we do...