Extract dask and spark test into distributed test. (#8395)

- Move test files.
- Run spark and dask separately to prevent conflicts.
- Gather common code into the testing module.

tests/python/test_tracker.py

@@ -7,6 +7,7 @@ import pytest
 import xgboost as xgb
 from xgboost import RabitTracker
 from xgboost import testing as tm
+from xgboost import collective
 
 if sys.platform.startswith("win"):
     pytest.skip("Skipping dask tests on Windows", allow_module_level=True)
@@ -21,12 +22,9 @@ def test_rabit_tracker():
 
 
 def run_rabit_ops(client, n_workers):
-    from test_with_dask import _get_client_workers
     from xgboost.dask import CommunicatorContext, _get_dask_config, _get_rabit_args
 
-    from xgboost import collective
-
-    workers = _get_client_workers(client)
+    workers = tm.get_client_workers(client)
     rabit_args = client.sync(_get_rabit_args, len(workers), _get_dask_config(), client)
     assert not collective.is_distributed()
     n_workers_from_dask = len(workers)
@@ -76,7 +74,6 @@ def test_rabit_ops_ipv6():
 
 def test_rank_assignment() -> None:
     from distributed import Client, LocalCluster
-    from test_with_dask import _get_client_workers
 
     def local_test(worker_id):
         with xgb.dask.CommunicatorContext(**args) as ctx:
@@ -89,7 +86,7 @@ def test_rank_assignment() -> None:
 
     with LocalCluster(n_workers=8) as cluster:
         with Client(cluster) as client:
-            workers = _get_client_workers(client)
+            workers = tm.get_client_workers(client)
             args = client.sync(
                 xgb.dask._get_rabit_args,
                 len(workers),