Makefile: Fix race in `no-proto-changes` target

The no-proto-changes target first regenerates the Protobuf files and then checks whether there are any changes compared to the Protobuf definitions. To regenerate the files, we first delete the target directory that contains the generated code so that we start from a clean slate. This is causing problems though when this target is executed via make verify, which besides other targets also executes the lint target. Now if the code is getting deleted while the linter runs in parallel then it may happen that linting fails due to the missing code.

This MR refactors our tests so that we can get rid of almost all of the test services thate exist in our codebase in favor of either using grpc_testing or our Gitaly services. The only remaining test service is moved into proto/testproto so that the generated test code is in the same tree as our production code. This buys us the ability to easily generate all Protobuf code in a different target directory so that we can just compare that overridden target directory with our committed sources. And that means that we don't ever have to touch the source directory to check for modifications, which fixes the race.