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_spark/test_data.py

@@ -1,13 +1,8 @@
 import sys
-from typing import List
 
-import numpy as np
-import pandas as pd
 import pytest
 
-sys.path.append("tests/python")
-
-import testing as tm
+from xgboost import testing as tm
 
 if tm.no_spark()["condition"]:
     pytest.skip(msg=tm.no_spark()["reason"], allow_module_level=True)
@@ -15,6 +10,7 @@ if sys.platform.startswith("win") or sys.platform.startswith("darwin"):
     pytest.skip("Skipping PySpark tests on Windows", allow_module_level=True)
 
 
+sys.path.append("tests/python")
 from test_spark.test_data import run_dmatrix_ctor
 
 

tests/python-gpu/test_gpu_spark/test_gpu_spark.py

@@ -6,8 +6,7 @@ import sys
 import pytest
 import sklearn
 
-sys.path.append("tests/python")
-import testing as tm
+from xgboost import testing as tm
 
 if tm.no_spark()["condition"]:
     pytest.skip(msg=tm.no_spark()["reason"], allow_module_level=True)