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_tree_regularization.py

@@ -1,8 +1,8 @@
 import numpy as np
-import xgboost as xgb
-
 from numpy.testing import assert_approx_equal
 
+import xgboost as xgb
+
 train_data = xgb.DMatrix(np.array([[1]]), label=np.array([1]))