Improve tests for `conf_validations`

[riley-harper]

The conf_validations module is not particularly well tested. We should at least have one test that has a valid config file so that we can confirm that all of the steps run without errors on a valid config file.

There are lots of edge cases and error conditions to be triggered during config validation. Trying to hit all of them is probably not worth it, at least for this issue, but hitting some of the more important or fragile ones may be helpful. It is helpful that the module is divided into several smallish functions, which should make it easier to test.