selftests: mptcp: add pm sysctl mapping tests

This patch checks if the newly added net.mptcp.path_manager is mapped
successfully from or to the old net.mptcp.pm_type in userspace_pm.sh.

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>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250313-net-next-mptcp-pm-ops-intro-v1-12-f4e4a88efc50@kernel.org


Signed-off-by: Paolo Abeni <pabeni@redhat.com>