extmod/asyncio: Fix early exit of asyncio scheduler.

This commit fixes three open issues related to the asyncio scheduler
exiting prematurely when the main task queue is empty, in cases where
CPython would not exit (for example, because the main task is not done
because it's on a different queue).

In the first case, the scheduler exits because running a task via
`run_until_complete` did not schedule any dependent tasks.

In the other two cases, the scheduler exits because the tasks are queued in
an event queue.

Tests have been added which reproduce the original issues.  These test
cases document the unauthorized use of `Event.set()` from a soft IRQ, and
are skipped in unsupported environments (webassembly and native emitter).

Fixes issues #16759, #16569 and #16318.

Signed-off-by: Yoctopuce dev <dev@yoctopuce.com>

tests/extmod/asyncio_event_queue.py.exp

@@ -0,0 +1,5 @@
+main started
+foo waiting
+asyncio still pending, unlocking event
+foo done
+main done