message: Improve coverage of server_lib.cc testing
  We add one more test method to cover the successful case. Testing now covers all lines except the LOG(ERROR) statement.
diff: @@ -14,11 +14,32 @@ limitations under the License.
  ==============================================================================*/
  #include "tensorflow/core/distributed_runtime/server_lib.h"
  +#include "tensorflow/core/lib/core/status_test_util.h"
  #include "tensorflow/core/lib/strings/str_util.h"
  #include "tensorflow/core/platform/test.h"
  namespace tensorflow {
  +class TestServerFactory : public ServerFactory {
  + public:
  + bool AcceptsOptions(const ServerDef& server_def) override {
  + return server_def.protocol() == "test_protocol";
  + }
  +
  + Status NewServer(const ServerDef& server_def,
  + std::unique_ptr<ServerInterface>* out_server) override {
  + return Status::OK();
  + }
  +};
  +
  +TEST(ServerLibTest, NewServerFactoryAccepts) {
  + ServerFactory::Register("TEST_SERVER", new TestServerFactory());
  + ServerDef server_def;
  + server_def.set_protocol("test_protocol");
  + std::unique_ptr<ServerInterface> server;
  + TF_EXPECT_OK(NewServer(server_def, &server));
  +}
  +
  TEST(ServerLibTest, NewServerNoFactoriesAccept) {
  ServerDef server_def;
  server_def.set_protocol("fake_protocol");
  