tests/extmod/asyncio_iterator_event.py: Use format instead of f-string.

Some targets don't have f-strings enabled, so try not to use them in tests.
Rather, use `str.format`, which is more portable.

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

tests/extmod/asyncio_iterator_event.py

@@ -50,7 +50,7 @@ def schedule_watchdog(end_ticks):
 async def test(ai):
     for x in range(3):
         await asyncio.sleep(0.1)
-        ai.fetch_data(f"bar {x}")
+        ai.fetch_data("bar {}".format(x))
 
 
 class AsyncIterable: