selftests: mptcp: lib: introduce 'nstat_{init,get}'

selftests: mptcp: lib: introduce 'nstat_{init,get}'

These new helpers are easier to read than the long and multi lines
commands. Plus it will ease the addition of new features related to that
in the next commits.

No behavioural changes intended.

Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20251114-net-next-mptcp-sft-count-cache-stats-timeout-v1-1-863cb04e1b7b@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>