Fix asyncio tests on Windows: wait for the subprocess exit

Before, regrtest failed to remove the temporary test directory because the
process was still running in this directory.

Lib/test/test_asyncio/test_windows_utils.py

@@ -164,6 +164,8 @@ class PopenTests(unittest.TestCase):
         self.assertTrue(msg.upper().rstrip().startswith(out))
         self.assertTrue(b"stderr".startswith(err))
 
+        p.wait()
+
 
 if __name__ == '__main__':
     unittest.main()