selftests: mptcp: join: update endpoint ops

This patch uses 'case' statements to simplify pm_nl_add_endpoint() and
pm_nl_check_endpoint(). And simplify pm_nl_check_endpoint() with
check_output() helper. Also update pm_nl_del_endpoint() to avoid the
'double quote' shellcheck warning.

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>