[dask] Extend tree stats tests. (#7128)

* Add tests to GPU.
* Assert cover in children sums up to the parent.

tests/python-gpu/test_gpu_with_dask.py

@@ -28,6 +28,7 @@ from test_with_dask import _get_client_workers        # noqa
 from test_with_dask import generate_array             # noqa
 from test_with_dask import kCols as random_cols       # noqa
 from test_with_dask import suppress                   # noqa
+from test_with_dask import run_tree_stats             # noqa
 import testing as tm                                  # noqa
 
 
@@ -493,6 +494,17 @@ class TestDistributedGPU:
         for rn, drn in zip(ranker_names, dranker_names):
             assert rn == drn
 
+    def test_tree_stats(self) -> None:
+        with LocalCUDACluster(n_workers=1) as cluster:
+            with Client(cluster) as client:
+                local = run_tree_stats(client, "gpu_hist")
+
+        with LocalCUDACluster(n_workers=2) as cluster:
+            with Client(cluster) as client:
+                distributed = run_tree_stats(client, "gpu_hist")
+
+        assert local == distributed
+
     def run_quantile(self, name: str, local_cuda_cluster: LocalCUDACluster) -> None:
         if sys.platform.startswith("win"):
             pytest.skip("Skipping dask tests on Windows")