remove usages of tonic-mock in tests

Opened by flokli at 2023-01-20T16·23+00 introduced tonic-mock, but as described in the commit message, upstream is a bit dead, and we're using brainrake's custom fork.

According to 1241164173, we might not need this crate at all, but can easily write some code for ourselves.

It might even happen naturally while writing easer-to-consume client library code.