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-gpu/test_from_cupy.py

@@ -1,12 +1,15 @@
-import numpy as np
-import xgboost as xgb
 import sys
+
+import numpy as np
 import pytest
 
+import xgboost as xgb
+
 sys.path.append("tests/python")
-import testing as tm
 from test_dmatrix import set_base_margin_info
 
+from xgboost import testing as tm
+
 
 def dmatrix_from_cupy(input_type, DMatrixT, missing=np.NAN):
     '''Test constructing DMatrix from cupy'''