[3.15] Add test for explicit cls argument bypassing default JSON decoder (GH-154094) (#154112)

Add test for explicit `cls` argument bypassing default JSON decoder (GH-154094)

* Add test for skipped `cls=None` case

* Fix lint

* Add new line between classes
(cherry picked from commit 5380589)

Co-authored-by: Dominic H <dom@dominic.sk>

Author: miss-islington

Lib/test/test_json/test_decode.py

@@ -1,4 +1,5 @@
 import decimal
+import unittest.mock
 from io import StringIO
 from collections import OrderedDict
 from test.test_json import PyTest, CTest
@@ -155,6 +156,15 @@ def test_limit_int(self):
             with self.assertRaises(ValueError):
                 self.loads('1' * (maxdigits + 1))
 
+    def test_explicit_cls_skips_json_decoder_default(self):
+        class CustomDecoder:
+            pass
+
+        with unittest.mock.patch.object(
+                CustomDecoder, 'decode', create=True) as mock_decode:
+            self.loads('{}', cls=CustomDecoder)
+        mock_decode.assert_called_once()
+
 
 class TestPyDecode(TestDecode, PyTest): pass
 class TestCDecode(TestDecode, CTest): pass