selftests: mptcp: move prefix tests of addr_nr_ns2 together

Move the fullmesh prefix test of addr_nr_ns2 together with its other
prefix tests.

Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>