I agree with you. If there is a defined Test of the Starstone then soon it could become the end goal of a majority adventuring parties in Golarion. I think this is best left to a DM to use their imagination and tailor as they see fit as they have characters that want to attempt the test.
As you mentioned if it gets written down and published then people will start to craft their ways through the challenges at the meta game level. If we leave it to the craftiness of the DMs the test can be whatever the DM sees fit. The unknown of the test can make it mean that much more to PCs in various campaigns.
It might be cool if it is very deadly and different for every person who enters. That way you can't learn your way through and would need to try the hard way (via destructive testing). The result would be a good thing, in my opinion, in making players think twice before entering.
One of my favorite things in the Pathfinder core rulebook was the recommendation not to do epic. The math for 3/3.5/PF works adequetely for 20 levels but begins to suffer issues after that! Godhood would be even worse.
That being said, if done correctly, it might be intriguing. BECMI had immortals rules and they looked very fun (never did get to use them).