Register loop shutdown on atexit

Lib/unittest/async_case.py

 import asyncio
+import atexit
 import inspect
 import warnings
 
@@ -240,6 +241,7 @@ def setUpAsyncioLoop(self):
         if loop is not None:
             loop = self.setUpGlobalAsyncioLoop()
             self.__class__._globalAsyncioLoop = loop
+            atexit.register(self.tearDownGlobalAsyncioLoop, loop)
         return loop
 
     def tearDownAsyncioLoop(self, loop):