selftests: mptcp: add evts_get_info helper

selftests: mptcp: add evts_get_info helper

This patch adds a new helper get_info_value(), using 'sed' command to
parse the value of the given item name in the line with the given keyword,
to make chk_mptcp_info() and pedit_action_pkts() more readable.

Also add another helper evts_get_info() to use get_info_value() to parse
the output of 'pm_nl_ctl events' command, to make all the userspace pm
selftests more readable, both in mptcp_join.sh and userspace_pm.sh.

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-v4-2-8d6b94150f6b@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>