unittest.mock test and coverage fixup (python#130787)

* Mark functions that will never be called with # pragma: no cover

* Fix testpatch.PatchTest.test_exit_idempotent

.stop() and __exit__ have subtly different code paths, so to really test __exit__ idempotency, we need to call it specifically twice.

Author: cjw296

Lib/test/test_unittest/testmock/testhelpers.py

@@ -1080,7 +1080,7 @@ def test_dataclass_with_method(self):
         class WithMethod:
             a: int
             def b(self) -> int:
-                return 1
+                return 1  # pragma: no cover
 
         for mock in [
             create_autospec(WithMethod, instance=True),

Lib/test/test_unittest/testmock/testmock.py

@@ -316,7 +316,7 @@ def test_explicit_return_value_even_if_mock_wraps_object(self):
         passed to the wrapped object and the return_value is returned instead.
         """
         def my_func():
-            return None
+            return None  # pragma: no cover
         func_mock = create_autospec(spec=my_func, wraps=my_func)
         return_value = "explicit return value"
         func_mock.return_value = return_value

Lib/test/test_unittest/testmock/testpatch.py

@@ -748,7 +748,7 @@ def test_stop_idempotent(self):
     def test_exit_idempotent(self):
         patcher = patch(foo_name, 'bar', 3)
         with patcher:
-            patcher.stop()
+            patcher.__exit__(None, None, None)
 
 
     def test_second_start_failure(self):