And you can contribute to it.
As far as I understand (and I’m just a Fedora user like you), some test are done automatically, but it’s unreasonable to expect they’ll find all the possible problems on all possible hardware configurations.
Apart from that, test are done by a (small?) group of community members on the hardware they own with the time they can afford for such test.
I haven’t ever heard about Fedora having a large hardware testing lab, or hardware testing partners, etc. I would be surprised (pleasantly) if I hear that’s the case.
So the most practical way of providing some more testing would be for ordinary users – such as you and me – to contribute to it. Please tell me if I’m wrong.
And making newer kernel packages stay in updates-testing longer won’t provide anything useful unless more users won’t test then. It will only delay their availability for most users. I.e. no positive effects, just more delay.
It’s quite hard to have both. For me personally Fedora strikes a near perfect balance, although I understand I take a risk of some update braking something (usually not much, and usually I manage to troubleshoot and maybe temporarily revert the change quite quickly). But I’d say that stable base is not really about Fedora, there are too many updates too quickly. That’s my opinion. And that’s what I like about Fedora (one of the things better to say) – updates providing bug and security fixes, and newer versions of application. And I accept the reverse side of this coin: updates can and sometimes will brake something for me.