selftests: mptcp: lib: use setup/cleanup_ns helpers

This patch includes lib.sh into mptcp_lib.sh, uses setup_ns helper
defined in lib.sh to set up namespaces in mptcp_lib_ns_init(), and
uses cleanup_ns to delete namespaces in mptcp_lib_ns_exit().

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/20240607-upstream-net-next-20240607-selftests-mptcp-net-lib-v1-5-e36986faac94@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>