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 <[e-mail]>
Reviewed-by: Geliang Tang <[e-mail]>
Signed-off-by: Matthieu Baerts (NGI0) <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

tools/testing/selftests/net/mptcp/mptcp_connect.sh

···
 	done
 fi
 
+if $capture; then
+	rndh="${ns1:4}"
+	mptcp_lib_pr_info "Packet capture files will have this prefix: ${rndh}-"
+fi
+
 set_ethtool_flags() {
 	local ns="$1"
 	local dev="$2"
···
 
 	if $capture; then
 		local capuser
-		local rndh="${connector_ns:4}"
 		if [ -z $SUDO_USER ] ; then
 			capuser=""
 		else