This change renames the pw_test_server module to pw_target_runner and
integrates it into the GN build using the new Go templates. The module
is updated to match the GOPATH structure under its go/ directory, and
its protos are moved into the module protos directory. Additionally,
the simple server and client are split into two separate programs
instead of combining them into one.
Change-Id: I44adeca0989dc7e15cb1a169a6ce183139dde2ce
