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 834bd5dd766cf212fb20d65d8a046c62a33006d4)

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