test: Refactor xpath handling in netconf/restconf

[mattiaswal]

I'm approving this, because I'm not sure now much of my feedback is reasonable to address now, but please have a think about it.

Great to finally get to protocol parity on the test side of things! :clap: :clap:

Originally posted by @wkz in https://github.com/kernelkit/infix/pull/484#pullrequestreview-2144428870