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>