Some tests for agentk

This MR adds only a small number of tests, focusing on the plumbing rather than tests itself. More will come later.

Mocking gRPC: https://github.com/grpc/grpc-go/blob/master/Documentation/gomock-example.md

gomock: https://github.com/golang/mock

go generate: https://blog.golang.org/generate, docs https://golang.org/cmd/go/#hdr-Generate_Go_files_by_processing_source