testhelper: Convert `GitalyOrPraefectMessage` to use generics

The helper function GitalyOrPraefectMessage() will return either one of two messages depending on whether we execute with Praefect as proxy or not. It is more generally useful than that though as we may also want to check other object types. And now that we can use generics it is needlessly tied to a specific object type.

Convert the function to accept any objects as input by using generics and rename it to GitalyOrPraefect().