Test Individual Validation Methods

[clenk]

From @gtback:

It would be great to test individual validation methods, in addition to (or possibly just in place of) doing a full validation. This came up when looking at the code that validates strict relationships. I'd like to avoid having tests that "succeed" but are passing a different validation check than what we expected. The checks are already pretty isolated in that they can mostly be called directly with instance data (and don't need ValidationOptions or the like).