selftests: mptcp: add mptcp_lib_check_transfer

selftests: mptcp: add mptcp_lib_check_transfer

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

check_transfer() and print_file_err() helpers are defined both in
mptcp_connect.sh and mptcp_sockopt.sh, export them into mptcp_lib.sh
and rename them with mptcp_lib_ prefix. And use them in all scripts.

Note: In mptcp_sockopt.sh it is OK to drop 'ret=1' in check_transfer()
because it will be set in run_tests() anyway.

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-v4-14-8d6b94150f6b@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>