One thing I want to point out about https://pubkit.net is that I know a service like this could be easily abused!
This is why you will have to verify ownership of servers you want to test against
It can be a text file or DNS record, and in special cases, I can make exceptions for devs who want to test against instances they can't publicly verify
The point is, PubKit will be opt-in, and won't require a fediblock, services like these can be done right 😎