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

@@ -1,4 +1,5 @@
 # Test get_event_loop()
+# Note: CPython deprecated get_event_loop() so this test needs a .exp
 
 try:
     import asyncio