selftests: mptcp: add mptcp_lib_get_counter

selftests: mptcp: add mptcp_lib_get_counter

To avoid duplicated code in different MPTCP selftests, we can add
and use helpers defined in mptcp_lib.sh.

The helper get_counter() in mptcp_join.sh and get_mib_counter() in
mptcp_connect.sh have the same functionality, export get_counter() into
mptcp_lib.sh and rename it as mptcp_lib_get_counter(). Use this new
helper instead of get_counter() and get_mib_counter().

Use this helper in test_prio() in userspace_pm.sh too instead of
open-coding.

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-v4-11-8d6b94150f6b@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>