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_gpu_linear.py

@@ -1,15 +1,10 @@
-import sys
-
 import pytest
 from hypothesis import assume, given, note, settings, strategies
 
 import xgboost as xgb
-from xgboost import testing
+from xgboost import testing as tm
 
-sys.path.append("tests/python")
-import testing as tm
-
-pytestmark = testing.timeout(10)
+pytestmark = tm.timeout(10)
 
 parameter_strategy = strategies.fixed_dictionaries({
     'booster': strategies.just('gblinear'),