Fix device communicator dependency (#9346)

tests/cpp/plugin/helpers.h

@@ -37,7 +37,14 @@ class ServerForTest {
   }
 
   ~ServerForTest() {
+    using namespace std::chrono_literals;
+    while (!server_) {
+      std::this_thread::sleep_for(100ms);
+    }
     server_->Shutdown();
+    while (!server_thread_) {
+      std::this_thread::sleep_for(100ms);
+    }
     server_thread_->join();
   }
 
@@ -56,7 +63,7 @@ class BaseFederatedTest : public ::testing::Test {
 
   void TearDown() override { server_.reset(nullptr); }
 
-  static int constexpr kWorldSize{3};
+  static int constexpr kWorldSize{2};
   std::unique_ptr<ServerForTest> server_;
 };