selftests: mptcp: add mptcp_lib_ns_init/exit helpers

Add helpers mptcp_lib_ns_init() and mptcp_lib_ns_exit() in mptcp_lib.sh
to initialize and delete the given namespaces. Then every test script
can invoke these helpers and use all namespaces.

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/20240306-upstream-net-next-20240304-selftests-mptcp-shared-code-shellcheck-v2-4-bc79e6e5e6a0@kernel.org


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