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_quantile_dmatrix.py

@@ -4,7 +4,7 @@ import numpy as np
 import pytest
 from hypothesis import given, settings, strategies
 from scipy import sparse
-from testing import (
+from xgboost.testing import (
     IteratorForTest,
     make_batches,
     make_batches_sparse,