selftests: mptcp: add mptcp_lib_make_file

selftests: mptcp: add mptcp_lib_make_file

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

make_file() helper in mptcp_sockopt.sh and userspace_pm.sh are the same.
Export it into mptcp_lib.sh and rename it as mptcp_lib_kill_wait(). Use
it in both mptcp_connect.sh and mptcp_join.sh.

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-v4-13-8d6b94150f6b@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>