Convert federated learner test into test suite. (#9018)

* Convert federated learner test into test suite.

- Add specialization to learning to rank.

tests/cpp/plugin/helpers.h

@@ -8,6 +8,7 @@
 #include <xgboost/json.h>
 
 #include <random>
+#include <thread>  // for thread, sleep_for
 
 #include "../../../plugin/federated/federated_server.h"
 #include "../../../src/collective/communicator-inl.h"
@@ -33,13 +34,17 @@ inline std::string GetServerAddress() {
 
 namespace xgboost {
 
-class BaseFederatedTest : public ::testing::Test {
- protected:
-  void SetUp() override {
+class ServerForTest {
+  std::string server_address_;
+  std::unique_ptr<std::thread> server_thread_;
+  std::unique_ptr<grpc::Server> server_;
+
+ public:
+  explicit ServerForTest(std::int32_t world_size) {
     server_address_ = GetServerAddress();
-    server_thread_.reset(new std::thread([this] {
+    server_thread_.reset(new std::thread([this, world_size] {
       grpc::ServerBuilder builder;
-      xgboost::federated::FederatedService service{kWorldSize};
+      xgboost::federated::FederatedService service{world_size};
       builder.AddListeningPort(server_address_, grpc::InsecureServerCredentials());
       builder.RegisterService(&service);
       server_ = builder.BuildAndStart();
@@ -47,15 +52,21 @@ class BaseFederatedTest : public ::testing::Test {
     }));
   }
 
-  void TearDown() override {
+  ~ServerForTest() {
     server_->Shutdown();
     server_thread_->join();
   }
+  auto Address() const { return server_address_; }
+};
+
+class BaseFederatedTest : public ::testing::Test {
+ protected:
+  void SetUp() override { server_ = std::make_unique<ServerForTest>(kWorldSize); }
+
+  void TearDown() override { server_.reset(nullptr); }
 
   static int const kWorldSize{3};
-  std::string server_address_;
-  std::unique_ptr<std::thread> server_thread_;
-  std::unique_ptr<grpc::Server> server_;
+  std::unique_ptr<ServerForTest> server_;
 };
 
 template <typename Function, typename... Args>