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/default.nix

@@ -64,7 +64,6 @@ let
         ) [
         "fuse-backend-rs"
         "test-generator"
-        "tonic-mock"
         "wu-manber"
       ]);
   };