tests/extmod: Add .exp test files for asyncio.get_event_loop tests.

And use `asyncio.new_event_loop()` where possible.  This change is needed
because CPython 3.12 deprecated the `get_event_loop()` function.
Signed-off-by: Damien George <damien@micropython.org>

tests/extmod/asyncio_get_event_loop.py

 # Test get_event_loop()
+# Note: CPython deprecated get_event_loop() so this test needs a .exp
 
 try:
     import asyncio

tests/extmod/asyncio_get_event_loop.py.exp

+start
+end

tests/extmod/asyncio_loop_stop.py

@@ -34,7 +34,7 @@ async def main():
     loop.stop()
 
 
-loop = asyncio.get_event_loop()
+loop = asyncio.new_event_loop()
 loop.create_task(main())
 
 for i in range(3):

tests/extmod/asyncio_new_event_loop.py

 # Test Loop.new_event_loop()
+# Note: CPython deprecated get_event_loop() so this test needs a .exp
 
 try:
     import asyncio

tests/extmod/asyncio_new_event_loop.py.exp

+start
+task 0
+stop
+start
+task 0
+stop