Add missing `MR::Model` unit tests

Not sure what happened, but there weren't any unit tests for the
`MR::Model` mixin. This adds them to ensure that there aren't any
coverage holes in MR's test suite.

@kellyredding - I probably meant to add these when I was switching out model logic for the mixins and just forgot. We were covered by system tests with non fake records but these still should've been in place.