Test on s390x. (#7038)

* Fix && remove unused parameter.

tests/python/test_basic.py

@@ -232,6 +232,7 @@ class TestBasic:
         assert isinstance(cv, dict)
         assert len(cv) == (4)
 
+    @pytest.mark.skipif(**tm.skip_s390x())
     def test_cv_explicit_fold_indices_labels(self):
         params = {'max_depth': 2, 'eta': 1, 'verbosity': 0, 'objective':
                   'reg:squarederror'}