In #10193 we introduced a testing helper that spins up a client RPC server
without the rest of the client operations so that we can make server-side client
RPC tests lighter. But this wasn't actually ever wired up to the intended
target. While working on Dynamic Host Volumes I noticed that this would be
useful for RPC tests.
This changeset fixes some bugs in the helper that arose from client code drift,
and makes it used by the client RPC tests for CSI. This will also get used for
the DHV RPC tests.
Ref: https://github.com/hashicorp/nomad/pull/10193
a8b84a6eed