selftests: mptcp: userspace pm send RM_ADDR for ID 0

This patch adds a selftest for userspace PM to remove id 0 address.

Use userspace_pm_add_addr() helper to add an id 10 address, then use
userspace_pm_rm_addr() helper to remove id 0 address.

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