tests/pull-corruption: Ensure we corrupt an object to be pulled

tests/pull-corruption: Ensure we corrupt an object to be pulled

This test had some nondeterminism because we chose a random
object to corrupt, but because there were multiple commits, it
was possible that we chose an object that was not being pulled.

Fix this by writing some custom GJS code to find an explicitly random
object that exists in a given ref, an change a random byte offset.
This adds a lot more randomness to the testing too.