remove usages of tonic-mock in tests

#243
Opened by flokli at 2023-01-20T16·23+00

https://cl.tvl.fyi/c/depot/+/7869 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 https://github.com/tyrchen/tonic-mock/pull/1#issuecomment- 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.