Modify the SafeHtml test to check the models directory

This fixes two things:

- The outcome of this test is no longer dependent
on which models have been loaded over the course
of the test suite.
- It is correctly scoped to checking just the models
in `app/models`, so that Mock objects do not have
the overhead of using SafeHtml and can be tested
in greater isolation (eg `traits/attachable_test.rb`)