selftests: mptcp: userspace pm address tests

This patch adds userspace pm tests support for mptcp_join.sh script. Add
userspace pm add_addr and rm_addr test cases in userspace_tests().

Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>