selftests: mptcp: diag: change timeout_poll to 30

selftests: mptcp: diag: change timeout_poll to 30

Even if it is set to 100ms from the beginning with commit
df62f2ec3df6 ("selftests/mptcp: add diag interface tests"), there is
no reason not to have it to 30ms like all the other tests. "diag.sh" is
not supposed to be slower than the other ones.

To maintain consistency with other scripts, this patch changes it to 30.

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://lore.kernel.org/r/20240223-upstream-net-next-20240223-misc-improvements-v1-8-b6c8a10396bd@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>