selftests: mptcp: add {get,format}_endpoint(s) helpers

The output formats of 'ip mptcp' commands are much different from that
of 'pm_nl_ctl' commands.

This patch adds a new helper format_endpoints() to format the outputs of
'ip mptcp' and 'pm_nl_ctl' with 'endpoints' arguments to hide these
differences.

A new helper named get_endpoint() has also been added to show a specific
endpoint identified by the given address ID, similar to mptcp_join.sh's
pm_nl_show_endpoints() helper, but showing all entries.

Use these two helpers in mptcp_join.sh and pm_netlink.sh to replace all
'pm_nl_ctl get' commands and outputs of 'pm_nl_ctl dump/get'.

Suggested-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
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>
Signed-off-by: David S. Miller <davem@davemloft.net>