selftests: mptcp: userspace pm get addr tests

This patch adds a new helper userspace_pm_get_addr() in mptcp_join.sh.
In it, parse the token value from the output of 'pm_nl_ctl events', then
pass it to pm_nl_ctl get_addr command. Use this helper in userspace pm
dump tests.

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>