Add ans6 blackhole server to notify system test

Add ans6 blackhole server to notify system test

On AlmaLinux 9 with Python 3.9, ans6 fails after TCP connection is
accepted but reading is blocked. See the block_reading() comment of the
asynserver, that this hack may not be universal, and do not run this
test on Enterprise Linux 9 with Python 3.9.

    2025-12-11 10:22:44,348    DEBUG  Accepted TCP connection from 10.53.0.3:38821
    2025-12-11 10:22:44,348     INFO  Blocking reads from 10.53.0.3:38821
    2025-12-11 10:22:44,348    DEBUG  Receiving TCP message length from 10.53.0.3:38821...
    2025-12-11 10:22:44,348    DEBUG  Received 1 TCP octets from 10.53.0.3:38821
    2025-12-11 10:22:45,144    DEBUG  Received UDP message: ae6c24000001000100000000076578616d706c650000060001c00c0006000100000000001d066d6e616d65310000000000040000012c0000012c001baf8000000e10
    2025-12-11 10:22:45,145     INFO  Received example/IN/SOA (ID=44652) query from 10.53.0.3:52759 (UDP)
    2025-12-11 10:22:45,145    DEBUG  [IN]
    [IN] id 44652
    [IN] opcode NOTIFY
    [IN] rcode NOERROR
    [IN] flags AA
    [IN] ;QUESTION
    [IN] example. IN SOA
    [IN] ;ANSWER
    [IN] example. 0 IN SOA mname1. . 4 300 300 1814400 3600
    [IN] ;AUTHORITY
    [IN] ;ADDITIONAL
    2025-12-11 10:22:45,145    DEBUG  Matched response handler: IgnoreAllQueries
    2025-12-11 10:22:45,145     INFO  Not sending a response to query (ID=44652) from 10.53.0.3:52759 (UDP)
    Traceback (most recent call last):
      File "/builds/isc-projects/bind9/bin/tests/system/notify_tmp_uja_xk9f/ans6/ans.py", line 27, in <module>
        main()
      File "/builds/isc-projects/bind9/bin/tests/system/notify_tmp_uja_xk9f/ans6/ans.py", line 23, in main
        server.run()
      File "/builds/isc-projects/bind9/bin/tests/system/isctest/asyncserver.py", line 170, in run
        asyncio.run(coroutine())
      File "/usr/lib64/python3.9/asyncio/runners.py", line 44, in run
        return loop.run_until_complete(main)
      File "/usr/lib64/python3.9/asyncio/base_events.py", line 647, in run_until_complete
        return future.result()
      File "/builds/isc-projects/bind9/bin/tests/system/isctest/asyncserver.py", line 183, in _run
        await self._work_done
    RuntimeError: Task was destroyed but it is pending!