Network updates only test the libraries, not terraform

[erikh]

tftest can re-apply settings to ensure that terraform has enforced them, but we're not using it properly. Not sure what I was thinking here.

Regardless, the TestNetworkUpdate test does not really test terraform at all; it tests that go-ztcentral works, which is nice and all but go-ztcentral ... already does that, so let's test that terraform enforces the plan instead.

I'll send anyone a beer who does this for me.