selftests: openvswitch: Catch cases where the tests are killed

In case of fatal signal, or early abort at least cleanup the current
test case.

Fixes: 25f16c87 ("selftests: add openvswitch selftest suite")
Signed-off-by: Aaron Conole <aconole@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>