selftests: mptcp: connect: cover splice mode

selftests: mptcp: connect: cover splice mode

The "splice" alternate mode for mptcp_connect.sh/.c is available now,
this patch adds mptcp_connect_splice.sh to test it in the MPTCP CI by
default.

Note that this mode is also supported by stable kernel versions, but
optimised in this patch series.

Suggested-by: Matthieu Baerts <matttbe@kernel.org>
Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260130-net-next-mptcp-splice-v2-6-31332ba70d7f@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>