aioble/multitests: Store a reference to tasks and cancel when done.

Storing references to tasks is required by CPython, and enforced by Ruff
RUF006.  In this case it's also reasonable to cancel these tasks once the
test is finished.

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

micropython/bluetooth/aioble/multitests/ble_write_order.py

@@ -44,12 +44,12 @@ async def instance0_task():
 
     # Register characteristic.written() handlers as asyncio background tasks.
     # The order of these is important!
-    asyncio.create_task(task_written(characteristic_second, "second"))
-    asyncio.create_task(task_written(characteristic_first, "first"))
+    task_second = asyncio.create_task(task_written(characteristic_second, "second"))
+    task_first = asyncio.create_task(task_written(characteristic_first, "first"))
 
     # This dummy task simulates background processing on a real system that
     # can block the asyncio loop for brief periods of time
-    asyncio.create_task(task_dummy())
+    task_dummy_ = asyncio.create_task(task_dummy())
 
     multitest.globals(BDADDR=aioble.config("mac"))
     multitest.next()
@@ -63,6 +63,10 @@ async def instance0_task():
 
         await connection.disconnected()
 
+    task_second.cancel()
+    task_first.cancel()
+    task_dummy_.cancel()
+
 
 async def task_written(chr, label):
     while True: