selftests: mptcp: connect: also cover alt modes

selftests: mptcp: connect: also cover alt modes

The "mmap" and "sendfile" alternate modes for mptcp_connect.sh/.c are
available from the beginning, but only tested when mptcp_connect.sh is
manually launched with "-m mmap" or "-m sendfile", not via the
kselftests helpers.

The MPTCP CI was manually running "mptcp_connect.sh -m mmap", but not
"-m sendfile". Plus other CIs, especially the ones validating the stable
releases, were not validating these alternate modes.

To make sure these modes are validated by these CIs, add two new test
programs executing mptcp_connect.sh with the alternate modes.

Fixes: 048d19d444be ("mptcp: add basic kselftest for mptcp")
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-v2-1-8230ddd82454@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>