remove usages of tonic-mock in tests
Opened by flokli at
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.