Move Python testing utilities into xgboost module. (#8379)

- Add typehints.
- Fixes for pylint.

Co-authored-by: Hyunsu Philip Cho <chohyu01@cs.washington.edu>

tests/python/test_ranking.py

@@ -1,13 +1,15 @@
-import numpy as np
-from scipy.sparse import csr_matrix
-import testing as tm
-import xgboost
-import os
 import itertools
+import os
 import shutil
 import urllib.request
 import zipfile
 
+import numpy as np
+from scipy.sparse import csr_matrix
+
+import xgboost
+from xgboost import testing as tm
+
 
 def test_ranking_with_unweighted_data():
     Xrow = np.array([1, 2, 6, 8, 11, 14, 16, 17])