@Sjogre
That’s kind of an unclear part, yeah. But my impression was that the test was whether Twilight can hatch the egg. (There was even a symbol like that on the side.)
Maybe one could ask this from Faust or someone who worked on that episode.
It definitely seemed like it was just a regular thing to do. It didn’t seem like it was some sort of special test for Twilight either, given that Celestia arrived late to the scene (and she was a filly for godssake).
Which of course raises the question why Spike is the only dragon in town.
Maybe ponies do hatch dragon eggs but then set them free? Part of the whole taking care of animals and nature job? Spike is just an exception who stuck around?
On the Spike & mother thing, I’d just take it from the dragon culture angle. There is no reason why dragons should do family matters the same way as ponies, except for doing it like that for the sake of kids watching this. I think it makes a lot more sense to portray dragons as noble, wild, proud. If the dragon way for example, is to leave the offspring to fend for himself until he can fly with the rest, that doesn’t mean Spike wouldn’t be interested in discovering the dragon traditions. Heck, I think he might even be flattering himself with the thought that he’s part of a tough-as-nails and proud breed. It’s all about his reconcilation of his two heritages. Even if he takes the family values from his pony half in the end, there is a lot he can learn and build into his identity, even if not everything is friendly-seeming.