Makefile: coverage: don't run tests twice

[thaJeztah]

• relates to Speed up "make coverage" #215
• relates to go {build,test}: rm -i option #2751

Makefile: coverage: don't run tests twice

Initially the tests were run twice, once with "-i" to "pre build" the tests;
8ebe435

Commit cb50952 removed the "-i" options,
as it's no longer needed since go1.10, however it did not remove the
(now duplicate) line, so tests were running twice.

Before this, every test was run twice:

After this, tests are only run once (cutting the time for the unit tests in ~half 😂)

[thaJeztah]

I have a patch for some of the other warnings as well (those also failed on macOS, but looks like CI ignores them);

protoc -I.:/home/circleci/.go_workspace/src/github.com/docker/swarmkit/vendor/github.com/gogo/protobuf:/home/circleci/.go_workspace/src/github.com/docker/swarmkit/vendor:/home/circleci/.go_workspace/src:/usr/local/go_workspace/src:/usr/local/include --include_imports --descriptor_set_out=/tmp/descriptors.pb-2068773414 --gogoswarm_out=plugins=grpc+deepcopy+storeobject+raftproxy+authenticatedwrapper,import_path=github.com/moby/swarmkit/v2/api,Mgoogle/protobuf/descriptor.proto=github.com/gogo/protobuf/protoc-gen-gogo/descriptor,Mgoogle/protobuf/field_mask.proto=github.com/gogo/protobuf/types,Mgoogle/protobuf/timestamp.proto=github.com/gogo/protobuf/types,Mgoogle/protobuf/duration.proto=github.com/gogo/protobuf/types,Mgoogle/protobuf/wrappers.proto=github.com/gogo/protobuf/types,Mgithub.com/docker/swarmkit/protobuf/plugin/plugin.proto=github.com/moby/swarmkit/v2/protobuf/plugin,Mgogoproto/gogo.proto=github.com/gogo/protobuf/gogoproto,Mgoogle/protobuf/any.proto=github.com/gogo/protobuf/types:/home/circleci/.go_workspace/src /home/circleci/.go_workspace/src/github.com/docker/swarmkit/api/ca.proto /home/circleci/.go_workspace/src/github.com/docker/swarmkit/api/control.proto /home/circleci/.go_workspace/src/github.com/docker/swarmkit/api/dispatcher.proto /home/circleci/.go_workspace/src/github.com/docker/swarmkit/api/health.proto /home/circleci/.go_workspace/src/github.com/docker/swarmkit/api/logbroker.proto /home/circleci/.go_workspace/src/github.com/docker/swarmkit/api/objects.proto /home/circleci/.go_workspace/src/github.com/docker/swarmkit/api/raft.proto /home/circleci/.go_workspace/src/github.com/docker/swarmkit/api/resource.proto /home/circleci/.go_workspace/src/github.com/docker/swarmkit/api/snapshot.proto /home/circleci/.go_workspace/src/github.com/docker/swarmkit/api/specs.proto /home/circleci/.go_workspace/src/github.com/docker/swarmkit/api/types.proto /home/circleci/.go_workspace/src/github.com/docker/swarmkit/api/watch.proto
/usr/local/go_workspace/src: warning: directory does not exist.
protoc -I.:/home/circleci/.go_workspace/src/github.com/docker/swarmkit/vendor/github.com/gogo/protobuf:/home/circleci/.go_workspace/src/github.com/docker/swarmkit/vendor:/home/circleci/.go_workspace/src:/usr/local/go_workspace/src:/usr/local/include --gogoswarm_out=plugins=grpc+deepcopy+storeobject+raftproxy+authenticatedwrapper,import_path=github.com/moby/swarmkit/v2/protobuf/plugin,Mgoogle/protobuf/field_mask.proto=github.com/gogo/protobuf/types,Mgoogle/protobuf/timestamp.proto=github.com/gogo/protobuf/types,Mgoogle/protobuf/duration.proto=github.com/gogo/protobuf/types,Mgoogle/protobuf/wrappers.proto=github.com/gogo/protobuf/types,Mgithub.com/docker/swarmkit/protobuf/plugin/plugin.proto=github.com/moby/swarmkit/v2/protobuf/plugin,Mgogoproto/gogo.proto=github.com/gogo/protobuf/gogoproto,Mgoogle/protobuf/any.proto=github.com/gogo/protobuf/types,Mgoogle/protobuf/descriptor.proto=github.com/gogo/protobuf/protoc-gen-gogo/descriptor:/home/circleci/.go_workspace/src /home/circleci/.go_workspace/src/github.com/docker/swarmkit/protobuf/plugin/plugin.proto
/usr/local/go_workspace/src: warning: directory does not exist.
protoc -I.:/home/circleci/.go_workspace/src/github.com/docker/swarmkit/vendor/github.com/gogo/protobuf:/home/circleci/.go_workspace/src/github.com/docker/swarmkit/vendor:/home/circleci/.go_workspace/src:/usr/local/go_workspace/src:/usr/local/include --gogoswarm_out=plugins=grpc+deepcopy+storeobject+raftproxy+authenticatedwrapper,import_path=github.com/moby/swarmkit/v2/protobuf/plugin/deepcopy/test,Mgoogle/protobuf/descriptor.proto=github.com/gogo/protobuf/protoc-gen-gogo/descriptor,Mgoogle/protobuf/field_mask.proto=github.com/gogo/protobuf/types,Mgoogle/protobuf/timestamp.proto=github.com/gogo/protobuf/types,Mgoogle/protobuf/duration.proto=github.com/gogo/protobuf/types,Mgoogle/protobuf/wrappers.proto=github.com/gogo/protobuf/types,Mgithub.com/docker/swarmkit/protobuf/plugin/plugin.proto=github.com/moby/swarmkit/v2/protobuf/plugin,Mgogoproto/gogo.proto=github.com/gogo/protobuf/gogoproto,Mgoogle/protobuf/any.proto=github.com/gogo/protobuf/types:/home/circleci/.go_workspace/src /home/circleci/.go_workspace/src/github.com/docker/swarmkit/protobuf/plugin/deepcopy/test/deepcopy.proto
/usr/local/go_workspace/src: warning: directory does not exist.
protoc -I.:/home/circleci/.go_workspace/src/github.com/docker/swarmkit/vendor/github.com/gogo/protobuf:/home/circleci/.go_workspace/src/github.com/docker/swarmkit/vendor:/home/circleci/.go_workspace/src:/usr/local/go_workspace/src:/usr/local/include --gogoswarm_out=plugins=grpc+deepcopy+storeobject+raftproxy+authenticatedwrapper,import_path=github.com/moby/swarmkit/v2/protobuf/plugin/raftproxy/test,Mgoogle/protobuf/timestamp.proto=github.com/gogo/protobuf/types,Mgoogle/protobuf/duration.proto=github.com/gogo/protobuf/types,Mgoogle/protobuf/wrappers.proto=github.com/gogo/protobuf/types,Mgithub.com/docker/swarmkit/protobuf/plugin/plugin.proto=github.com/moby/swarmkit/v2/protobuf/plugin,Mgogoproto/gogo.proto=github.com/gogo/protobuf/gogoproto,Mgoogle/protobuf/any.proto=github.com/gogo/protobuf/types,Mgoogle/protobuf/descriptor.proto=github.com/gogo/protobuf/protoc-gen-gogo/descriptor,Mgoogle/protobuf/field_mask.proto=github.com/gogo/protobuf/types:/home/circleci/.go_workspace/src /home/circleci/.go_workspace/src/github.com/docker/swarmkit/protobuf/plugin/raftproxy/test/service.proto
/usr/local/go_workspace/src: warning: directory does not exist.
protoc -I /usr/local/include --decode google.protobuf.FileDescriptorSet /usr/local/include/google/protobuf/descriptor.proto

[thaJeztah]

Moved the fix for the failing test to #3087 and removed the second commit (which wasn't needed after all).

@corhere @neersighted @dperny PTAL

[thaJeztah]

Thx! Let me bring this one in