test_send_timeout check can be unstable

Sometimes the test_send_timeout check of the timeouts system test fails by not raising the EOFError exception after filling the TCP send buffer:

[gw3] linux -- Python 3.11.3 /usr/bin/python3
/builds/isc-private/bind9/bin/tests/system/timeouts/tests_tcp_timeouts.py:206: in test_send_timeout
    with pytest.raises(EOFError):
E   Failed: DID NOT RAISE <class 'EOFError'>

I think the TCP send buffer hasn't been filled, and the guarded send_tcp succeeded where ConnectionResetError was expected.

There's nothing substantive in the ns1/named.run log because named starts with -d 1.