The temple ship's Ethereals complain as you kill them that "this is not your path!" The test was not to kill the uber ethereal. If the test was more than a desperate gambit, which I still don't believe, then the goal was to reach the Uber Ethereal and join them.
The aliens don't need XCOM's volunteer, they already utilize powerful human psionics. Which is why they do everything they can to end the XCOM project while keeping humanity alive. They even release chryssalids on cities to erode support for XCOM. If XCOM doesn't stop those chryssalids, the nation leaves the council. Probably because it's *swarming with chryssalids*. The aliens are practically killing off whole nations, damaging their prize, because it's the only way to stop XCOM. That's not a "test", that's them getting desperate.
It's true they start with smaller ships and weaker soldiers. Besides gameplay/storyline segregation, this makes sense if they didn't see humanity's psi potential at first. Or understand how effectively we'd resist them. They act like their resources are limited, so they probably are. Consider the number of intact ships we capture, including the Slingshot battleship. Yet no mention of FTL drives, even as theoretical technology we can't figure out soon. The implication is that the temple ship is producing a wormhole, or psionic "rift". Easily explains the limited number of UFOs, and why its destruction is considered a victory.
Additionally, and this is speculation, I bet the temple ship itself doesn't have a FTL drive either. It might have had to fly here conventionally, explaining why it stuck around to be boarded. It also provides a way for XCOM 2 to be won: Like in XCOM 1, X-COM 1, and Half Life 2, the alien occupation can be cut off from their empire for a loooong time.
Dr Chen was right that the aliens could destroy humanity easily, but not that they could conquer.