refactor(tvix/castore): move tests to grpc client, rm tonic-mock

Similar to gen_directorysvc_grpc_client, introduce a
gen_blobsvc_grpc_client function that provides a gRPC client connected
to a blobservice.

The test is update to use that client to test against, rather than the
server trait, removing the last usage of tonic_mock, so it's removed
as well.

Fixes b/243.

Change-Id: If746e8600588da247eb53a63b70fe72f139e9e77
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9564
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>
Reviewed-by: Connor Brewster <cbrewster@hey.com>
Autosubmit: flokli <flokli@flokli.de>

tvix/Cargo.lock

@@ -2626,19 +2626,6 @@ dependencies = [
  "syn 1.0.109",
 ]
 
-[[package]]
-name = "tonic-mock"
-version = "0.1.0"
-source = "git+https://github.com/brainrake/tonic-mock?branch=bump-dependencies#ec1a15510875de99d709d684190db5d9beab175e"
-dependencies = [
- "bytes",
- "futures",
- "http",
- "http-body",
- "prost",
- "tonic",
-]
-
 [[package]]
 name = "tonic-reflection"
 version = "0.5.0"
@@ -2796,7 +2783,6 @@ dependencies = [
  "tokio-util",
  "tonic",
  "tonic-build",
- "tonic-mock",
  "tonic-reflection",
  "tower",
  "tracing",