selftests: mptcp: run mptcp_inq from a clean netns

A new "sandbox" net namespace is available where no other netfilter
rules have been added.

Use this new netns instead of re-using "ns1" and clean it.

Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>