tests/extmod: Make get_event_loop tests compatible with CPython 3.12.

Follow up to 2e852522b1: instead of having
.exp files for the get_event_loop tests, tweak them so they are compatible
with CPython 3.12.  This requires calling `asyncio.set_event_loop()` so
there is an active event loop and `asyncio.get_event_loop()` succeeds
without a warning.

Signed-off-by: Damien George <damien@micropython.org>

tests/extmod/asyncio_get_event_loop.py

@@ -1,5 +1,4 @@
 # Test get_event_loop()
-# Note: CPython deprecated get_event_loop() so this test needs a .exp
 
 try:
     import asyncio
@@ -7,6 +6,11 @@ except ImportError:
     print("SKIP")
     raise SystemExit
 
+# CPython 3.12 deprecated calling get_event_loop() when there is no current event
+# loop, so to make this test run on CPython requires setting the event loop.
+if hasattr(asyncio, "set_event_loop"):
+    asyncio.set_event_loop(asyncio.new_event_loop())
+
 
 async def main():
     print("start")

tests/extmod/asyncio_get_event_loop.py.exp

@@ -1,2 +0,0 @@
-start
-end

tests/extmod/asyncio_new_event_loop.py

@@ -1,5 +1,4 @@
 # Test Loop.new_event_loop()
-# Note: CPython deprecated get_event_loop() so this test needs a .exp
 
 try:
     import asyncio
@@ -7,6 +6,11 @@ except ImportError:
     print("SKIP")
     raise SystemExit
 
+# CPython 3.12 deprecated calling get_event_loop() when there is no current event
+# loop, so to make this test run on CPython requires setting the event loop.
+if hasattr(asyncio, "set_event_loop"):
+    asyncio.set_event_loop(asyncio.new_event_loop())
+
 
 async def task():
     for i in range(4):

tests/extmod/asyncio_new_event_loop.py.exp

@@ -1,6 +0,0 @@
-start
-task 0
-stop
-start
-task 0
-stop