tests/sys/netgraph: Further CI fixes

[arichardson]

Summary

I was trying to debug why this test is working locally but failing in CI.
While doing so I made some small changes to allow running it with set -e.

It turns out the problem is that find_iface does not return anything in
Jenkins, so all following tests fail with obscure error messages.
To handle this case exit early if $eth is empty.

[lwhsu]

I'm thinking if we should skip the tests needs an ether interface or create an interface in setup.