[coll] Add comm group. (#9759)

- Implement `CommGroup` for double dispatching.
- Small cleanup to tracker for handling abort.

tests/cpp/collective/test_worker.h

@@ -95,7 +95,8 @@ void TestDistributed(std::int32_t n_workers, WorkerFn worker_fn) {
   std::chrono::seconds timeout{1};
 
   std::string host;
-  ASSERT_TRUE(GetHostAddress(&host).OK());
+  auto rc = GetHostAddress(&host);
+  ASSERT_TRUE(rc.OK()) << rc.Report();
   RabitTracker tracker{StringView{host}, n_workers, 0, timeout};
   auto fut = tracker.Run();