Replace AER tests with ivreg

[nfultz]

AER::ivreg has been superseded with a new package; the tests here and DESCRIPTION Suggests will eventually need to be updated to reflect that.

Per https://stat.ethz.ch/pipermail/r-packages/2020/001727.html

[lukesonnet]

Going to wait for what I believe is a bug to be fixed in their diagnostics before landing this (see: https://github.com/john-d-fox/ivreg/issues/8)