What makes you think that the first attack was some sort of a "test"? Surely the intent was to exterminate mankind there and then, and the weapon seemed to be perfectly capable of doing just that - right until the point where it malfunctioned.
Memory Alpha makes up all sorts of nonsense about the weapon being a "probe" that "self-destructed" after a successful mission, but none of that appears backed up by the actual aired stories. The Xindi just built a weapon intended to destroy Earth, and it failed; they built a second one, and Archer took it away from them; and they built a third one, and it either succeeded ("Twilight") or was thwarted at the last minute ("Zero Hour").
Now, it's quite possible that the second weapon was just a test rig for the third one. But nothing suggests that the first one would have been a test rig, a prototype, or a shot in the dark. It was probably a well tested system with various prototypes preceding it - and despite all this, it just didn't deliver.