'TestService' in test.proto

- make the directory structure match the package structure
- sanitize the package structure (test --> testing)
- add auto-detection of proto file dependencies

(via ahh@google.com)
Under some configurations gRPC includes google3 base headers.  google3
has a namespace "thread" which mixes poorly with unqualified use of the
type std::thread; adding that namespace to various google3 headers
breaks this test.