test: fix the xi2 protocol swapping tests to actually work

This tests override WriteToClient() with their own custom function to check for validity. Unfortunately they also papered over bugs - to compare values we had to swap back thus modifying the original reply.

This doesn't really have an effect on most reply handling but for those with extra data it may affect how they are processed. Fix this by copying the reply so any of the fields within that we swap is left as-is and put some basic sanity checks in for the length we pass in.