selftests: mptcp: join: allow more time to send ADD_ADDR

selftests: mptcp: join: allow more time to send ADD_ADDR

When many ADD_ADDR need to be sent, it can take some time to send each
of them, and create new subflows. Some CIs seem to occasionally have
issues with these tests, especially with "debug" kernels.

Two subtests will now run for a slightly longer time: the last two where
3 or more ADD_ADDR are sent during the test.

Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250907-net-next-mptcp-add_addr-retrans-adapt-v1-3-824cc805772b@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>