[fed] Split up federated test CMake file. (#10566)

- Collect all federated test files into the same directory.
- Independently list the files.

tests/cpp/plugin/federated/CMakeLists.txt

@@ -0,0 +1,20 @@
+target_sources(
+  testxgboost PRIVATE
+  ${xgboost_SOURCE_DIR}/tests/cpp/plugin/federated/test_federated_coll.cc
+  ${xgboost_SOURCE_DIR}/tests/cpp/plugin/federated/test_federated_comm.cc
+  ${xgboost_SOURCE_DIR}/tests/cpp/plugin/federated/test_federated_comm_group.cc
+  ${xgboost_SOURCE_DIR}/tests/cpp/plugin/federated/test_federated_tracker.cc
+  ${xgboost_SOURCE_DIR}/tests/cpp/plugin/federated/test_federated_learner.cc
+  ${xgboost_SOURCE_DIR}/tests/cpp/plugin/federated/test_federated_data.cc
+)
+
+if(USE_CUDA)
+  target_sources(
+    testxgboost PRIVATE
+    ${xgboost_SOURCE_DIR}/tests/cpp/plugin/federated/test_federated_coll.cu
+    ${xgboost_SOURCE_DIR}/tests/cpp/plugin/federated/test_federated_comm_group.cu
+  )
+endif()
+
+target_include_directories(testxgboost PRIVATE ${xgboost_SOURCE_DIR}/plugin/federated)
+target_link_libraries(testxgboost PRIVATE federated_client)

tests/cpp/plugin/federated/test_federated_data.cc

@@ -1,15 +1,13 @@
-/*!
- * Copyright 2023 XGBoost contributors
+/**
+ * Copyright 2023-2024, XGBoost contributors
  */
 #include <gtest/gtest.h>
 #include <xgboost/data.h>
 
-#include <thread>
-
-#include "../../../src/collective/communicator-inl.h"
-#include "../filesystem.h"
-#include "../helpers.h"
-#include "federated/test_worker.h"
+#include "../../../../src/collective/communicator-inl.h"
+#include "../../filesystem.h"
+#include "../../helpers.h"
+#include "test_worker.h"
 
 namespace xgboost {
 

tests/cpp/plugin/federated/test_federated_learner.cc

@@ -1,19 +1,18 @@
 /**
  * Copyright 2023-2024, XGBoost contributors
  *
- * Some other tests for federated learning are in the main test suite (test_learner.cc),
- * gaurded by the `XGBOOST_USE_FEDERATED`.
+ * Some other tests for federated learning are in the main test suite (test_learner.cc).
  */
 #include <dmlc/parameter.h>
 #include <gtest/gtest.h>
 #include <xgboost/data.h>
 #include <xgboost/objective.h>
 
-#include "../../../src/collective/communicator-inl.h"
-#include "../../../src/common/linalg_op.h"  // for begin, end
-#include "../helpers.h"
-#include "../objective_helpers.h"  // for MakeObjNamesForTest, ObjTestNameGenerator
-#include "federated/test_worker.h"
+#include "../../../../src/collective/communicator-inl.h"
+#include "../../../../src/common/linalg_op.h"  // for begin, end
+#include "../../helpers.h"
+#include "../../objective_helpers.h"  // for MakeObjNamesForTest, ObjTestNameGenerator
+#include "test_worker.h"
 
 namespace xgboost {
 namespace {