Edit: We also don't know where this test will be held and how long it takes. Hopefully the temple of trials isn't two days away and Tlalli will wonder where we went. Or Tlalli comes back while our seven days of desert survival are ongoing and we have to tell our nomad friends that their traditions are all nice and interesting but we have more important things on our plate right now.
So your worst case scenario is that there happens to be a temple nearby for their traditional challenge and we go exploring it? Basically that we'd end up on an interesting sidequest when we're not actually under a deadline to get back? That's not too bad. And that scenario isn't even likely, considering that nomads as a rule are probably more fond of traditions that they can execute wherever.
No, worst case scenario is that we lose Tlallis company forever and are stuck on an unknown level of the tower making a three week trip back home into an odyssey where we are just as likely to end up five levels higher if we take the wrong turn. This sounds somewhat exciting actually. But I prefer that we had Rain with us, be at least somewhat prepared for such a trek and have a trainer or guide with us.
This is based on your fanciful scenario that our trial for the right to stay at the oasis involves us leaving the oasis so much as to defeat the purpose of taking the trial in the first place.
Second bad scenario is where we are going to piss off the nomads in a major way, Tlalli has to bail us out and will treat us for the rest of the way as dumb little children and fuck off the moment Scotsfield comes into view.
That would be if we picked "make me" or "attack the nomads" as our options. Failing a trial won't exactly offend them. It'll be no less than they expect.
Makes you wonder if we would have had Rain with us the entire time, if we took her to the ruins.
With her own Astra.
Eh, I suspect Rasputin would've taken an interest in her, and that the tunnels might've been more dangerous, but it's hard to say. Right now I'm just glad that we chose to go with the girls and that we're in charge here, as opposed to being stuck with the twins or Andrew's group.
Any kind of challenge which rely on knowledge that's learned by living in the desert?
Sure, a knowledge quiz would be one of the worse ones for us, but even then we wouldn't be out of luck. And if it's a more practical demonstration of capability they're after, we're even better off. Even trials such as you suggest would be within our abilities.
Like finding the good place to camp,
Decent odds we can do a passable job of this, between our party's astras and our MC's own sense for camping.
how to orient yourselves in the desert,
We have a magic map.
how to see a sandstorm coming,
Our girl has magic glasses that are rather good at seeing things coming.
tracking someone/something,
We have experience tracking prey, and Sophie has magic glasses. Trails might be different, but we wouldn't be clueless here.
which animals/plants are to edible/poisonous...
We have wilderness experience and our rank of alchemy just went up by 1.
Or it's even a quiz on the legends/stories of the tribe, so we're SOL
That would be a trial for becoming their storyteller or loremaster, not so much the trial to be considered a functional member of their tribe.