chore(tvix/store): add tonic-mock

Upstream seems to be dead, so we're using https://github.com/tyrchen/
tonic-mock/pull/3 here.

According to https://github.com/tyrchen/tonic-mock/pull/1#issuecomment-
1241164173, we might not need this crate at all, but for now, it gets
the job done and is less code to write in the tests.

Change-Id: Ia77fa19b998a5bbabd0311cc714b85a2ee30f36a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7869
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/store/Cargo.toml

@@ -32,6 +32,7 @@ tonic-build = "0.8.2"
 [dev-dependencies]
 test-case = "2.2.2"
 tempfile = "3.3.0"
+tonic-mock = { git = "https://github.com/brainrake/tonic-mock", branch = "bump-dependencies" }
 
 [features]
 default = [