selftests: mptcp: add mptcp_lib_check_tools helper

This patch exports check_tools() helper from mptcp_join.sh into
mptcp_lib.sh as a public one mptcp_lib_check_tools(). The arguments
"ip", "ss", "iptables" and "ip6tables" are passed into this helper
to indicate whether to check ip tool, ss tool, iptables and ip6tables
tools.

This helper can be used in every scripts.

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/20240306-upstream-net-next-20240304-selftests-mptcp-shared-code-shellcheck-v2-2-bc79e6e5e6a0@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>