selftests: rtnetlink: remove testns on test fail

This patch removes testns after test failure so that next test can
continue with clean ns

Signed-off-by: Prashant Bhole <bhole_prashant_q7@lab.ntt.co.jp>
Acked-by: William Tu <u9012063@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>