selftests: mptcp: disable add_addr retrans in endpoint_tests

To prevent test instability in the "delete re-add signal" test caused by
ADD_ADDR retransmissions, disable retransmissions for this test by setting
net.mptcp.add_addr_timeout to 0.

Suggested-by: Matthieu Baerts <matttbe@kernel.org>
Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250815-net-mptcp-misc-fixes-6-17-rc2-v1-6-521fe9957892@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Geliang Tang

tools/testing/selftests/net/mptcp/mptcp_join.sh

@@ -3842,6 +3842,7 @@ endpoint_tests()
 	# remove and re-add
 	if reset_with_events "delete re-add signal" &&
 	   mptcp_lib_kallsyms_has "subflow_rebuild_header$"; then
+		ip netns exec $ns1 sysctl -q net.mptcp.add_addr_timeout=0
 		pm_nl_set_limits $ns1 0 3
 		pm_nl_set_limits $ns2 3 3
 		pm_nl_add_endpoint $ns1 10.0.2.1 id 1 flags signal