selftests: mptcp: connect: print pcap prefix

To be able to find which capture files have been produced after several
runs.

This prefix was not printed anywhere before.

While at it, always use the same prefix by taking info from ns1, instead
of "$connector_ns", which is sometimes ns1, sometimes ns2 in the
subtests.

Reviewed-by: Mat Martineau <martineau@kernel.org>
Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250912-net-mptcp-fix-sft-connect-v1-5-d40e77cbbf02@kernel.org


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