gh-95010: Fix asyncio GenericWatcherTests.test_create_subprocess_fails_with_inactive_watcher (GH-95009)

The test was never run, because it was missing the TestCase class.
The test failed because the wrong attribute was patched.
(cherry picked from commit 834bd5dd76)

Co-authored-by: Thomas Grainger <tagrain@gmail.com>

Lib/test/test_asyncio/test_subprocess.py

@@ -700,6 +700,32 @@ class SubprocessPidfdWatcherTests(SubprocessWatcherMixin,
                                       test_utils.TestCase):
         Watcher = unix_events.PidfdChildWatcher
 
+
+    class GenericWatcherTests(test_utils.TestCase):
+
+        def test_create_subprocess_fails_with_inactive_watcher(self):
+            watcher = mock.create_autospec(
+                asyncio.AbstractChildWatcher,
+                **{"__enter__.return_value.is_active.return_value": False}
+            )
+
+            async def execute():
+                asyncio.set_child_watcher(watcher)
+
+                with self.assertRaises(RuntimeError):
+                    await subprocess.create_subprocess_exec(
+                        os_helper.FakePath(sys.executable), '-c', 'pass')
+
+                watcher.add_child_handler.assert_not_called()
+
+            with asyncio.Runner(loop_factory=asyncio.new_event_loop) as runner:
+                self.assertIsNone(runner.run(execute()))
+            self.assertListEqual(watcher.mock_calls, [
+                mock.call.__enter__(),
+                mock.call.__enter__().is_active(),
+                mock.call.__exit__(RuntimeError, mock.ANY, mock.ANY),
+            ])
+
 else:
     # Windows
     class SubprocessProactorTests(SubprocessMixin, test_utils.TestCase):
@@ -710,25 +736,5 @@ def setUp(self):
             self.set_event_loop(self.loop)
 
 
-class GenericWatcherTests:
-
-    def test_create_subprocess_fails_with_inactive_watcher(self):
-
-        async def execute():
-            watcher = mock.create_authspec(asyncio.AbstractChildWatcher)
-            watcher.is_active.return_value = False
-            asyncio.set_child_watcher(watcher)
-
-            with self.assertRaises(RuntimeError):
-                await subprocess.create_subprocess_exec(
-                    os_helper.FakePath(sys.executable), '-c', 'pass')
-
-            watcher.add_child_handler.assert_not_called()
-
-        self.assertIsNone(self.loop.run_until_complete(execute()))
-
-
-
-
 if __name__ == '__main__':
     unittest.main()