selftests: mptcp: extract mptcp_lib_check_expected

Extract the main part of check_expected() in userspace_pm.sh to a new
function mptcp_lib_check_expected() in mptcp_lib.sh. It will be used
in both mptcp_john.sh and userspace_pm.sh. check_expected_one() is
moved into mptcp_lib.sh too as mptcp_lib_check_expected_one().

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>
Link: https://lore.kernel.org/r/20240308-upstream-net-next-20240308-selftests-mptcp-unification-v1-11-4f42c347b653@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>