selftests: mptcp: add mptcp_lib_is_v6

selftests: mptcp: add mptcp_lib_is_v6

To avoid duplicated code in different MPTCP selftests, we can add
and use helpers defined in mptcp_lib.sh.

is_v6() helper is defined in mptcp_connect.sh, mptcp_join.sh and
mptcp_sockopt.sh, so export it into mptcp_lib.sh and rename it as
mptcp_lib_is_v6(). Use this new helper in all scripts.

Reviewed-by: Matthieu Baerts <matttbe@kernel.org>
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <martineau@kernel.org>
Link: https://lore.kernel.org/r/20231128-send-net-next-2023107-v4-10-8d6b94150f6b@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>