python/tests: make the test_assoc_group_fail2() test more resilient against timing

python/tests: make the test_assoc_group_fail2() test more resilient against timing

On a busy system [e]poll() on the server will mark both the
old connection fd and also the listening fd as readable.

epoll() returns the events in order, so the server processes the
disconnect first.

With poll() we don't have an order of the events and the
server is likely to process the connect before the disconnect.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>