selftests: mptcp: connect: also cover checksum

selftests: mptcp: connect: also cover checksum

The checksum mode has been added a while ago, but it is only validated
when manually launching mptcp_connect.sh with "-C".

The different CIs were then not validating these MPTCP Connect tests
with checksum enabled. To make sure they do, add a new test program
executing mptcp_connect.sh with the checksum mode.

Fixes: 94d66ba1d8e4 ("selftests: mptcp: enable checksum in mptcp_connect.sh")
Cc: stable@vger.kernel.org
Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250715-net-mptcp-sft-connect-alt-v2-2-8230ddd82454@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>