test: gRPC leaks go routine

[xiang90]

Too many goroutines running after all test(s).
1 instances of:
sync.(*Mutex).Unlock(...)
	/usr/local/golang/1.7.5/go/src/sync/mutex.go:102
github.com/coreos/pkg/capnslog.(*PackageLogger).internalLog(...)
	/home/runner/workspace/src/github.com/coreos/etcd/gopath/src/github.com/coreos/pkg/capnslog/pkg_logger.go:38 +0xda
github.com/coreos/pkg/capnslog.(*PackageLogger).Logf(...)
	/home/runner/workspace/src/github.com/coreos/etcd/gopath/src/github.com/coreos/pkg/capnslog/pkg_logger.go:48 +0x141
github.com/coreos/pkg/capnslog.(*PackageLogger).Printf(...)
	/home/runner/workspace/src/github.com/coreos/etcd/gopath/src/github.com/coreos/pkg/capnslog/pkg_logger.go:63 +0x70
google.golang.org/grpc/grpclog.Printf(...)
	/home/runner/workspace/src/github.com/coreos/etcd/gopath/src/google.golang.org/grpc/grpclog/logger.go:87 +0x89
google.golang.org/grpc.(*addrConn).resetTransport(...)
	/home/runner/workspace/src/github.com/coreos/etcd/gopath/src/google.golang.org/grpc/clientconn.go:698 +0x6a4
google.golang.org/grpc.(*ClientConn).resetAddrConn.func1(...)
	/home/runner/workspace/src/github.com/coreos/etcd/gopath/src/google.golang.org/grpc/clientconn.go:498 +0x42
created by google.golang.org/grpc.(*ClientConn).resetAddrConn
	/home/runner/workspace/src/github.com/coreos/etcd/gopath/src/google.golang.org/grpc/clientconn.go:507 +0x643
FAIL	github.com/coreos/etcd/clientv3	7.920s

Not sure if this is already reported.

[heyitsanthony]

Seems to be fixed by #7500. Closing.