extmod/uasyncio: Add clear method to ThreadSafeFlag.

This is useful in situations where the ThreadSafeFlag is reused and needs
to be cleared of any previous, unwanted event.

For example, clear the flag at the start of an operation, trigger the
operation (eg an I2C write), then (a)wait for an external event to set the
flag (eg a pin IRQ).  Further events may trigger the flag again but these
are unwanted and should be cleared before the next cycle starts.

tests/extmod/uasyncio_threadsafeflag.py

@@ -75,5 +75,25 @@ async def main():
     print("wait task")
     await t
 
+    # Flag set, cleared, and set again.
+    print("----")
+    print("set event")
+    flag.set()
+    print("yield")
+    await asyncio.sleep(0)
+    print("clear event")
+    flag.clear()
+    print("yield")
+    await asyncio.sleep(0)
+    t = asyncio.create_task(task(4, flag))
+    print("yield")
+    await asyncio.sleep(0)
+    print("set event")
+    flag.set()
+    print("yield")
+    await asyncio.sleep(0)
+    print("wait task")
+    await t
+
 
 asyncio.run(main())

tests/extmod/uasyncio_threadsafeflag.py.exp

@@ -19,3 +19,14 @@ yield
 task 3
 task 3 done
 wait task
+----
+set event
+yield
+clear event
+yield
+yield
+task 4
+set event
+yield
+wait task
+task 4 done