test: improve nodeserver testing

Cleanup some variable names in tests and split success/failure into
separate test cases when it seems like the expectation logic should
be different
(https://github.com/golang/go/wiki/TestComments#table-driven-tests-vs-multiple-test-functions).

I also updated this to use a mock provider grpc server like we use
in the rotation reconciler.

This improves the package test coverage from 55.8% -> 56.9%.