Mark test_send_timeout as flaky

In some cases, BIND is not fast enough to fill the send buffer and
manages to answer all queries, contrary to what the test expects.
Repeat the check up to 3 times to limit this test instability.

(cherry picked from commit 681b23c398)

bin/tests/system/timeouts/tests_tcp_timeouts.py

@@ -185,6 +185,7 @@ def test_long_axfr(named_port):
         assert soa is not None
 
 
+@pytest_custom_markers.flaky(max_runs=3)
 def test_send_timeout(named_port):
     with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as sock:
         sock.connect(("10.53.0.1", named_port))