[3.10] gh-91676 gh-91260 unittest.IsolatedAsyncioTestCase no longer leaks its executor (GH-91680)

For things like test_asyncio.test_thread this was causing frequent
"environment modified by test" errors as the executor threads had not
always stopped running after the test was over.

Lib/unittest/async_case.py

@@ -148,6 +148,8 @@ def _tearDownAsyncioLoop(self):
             # shutdown asyncgens
             loop.run_until_complete(loop.shutdown_asyncgens())
         finally:
+            # Prevent our executor environment from leaking to future tests.
+            loop.run_until_complete(loop.shutdown_default_executor())
             asyncio.set_event_loop(None)
             loop.close()