Fix HasManyThroughAssociationsTest tests

At the moment HasManyThroughAssociationsTest are failing, 1 failure and 1 error, when running just that test file, NOT when running all tests.

The failure is in method "test_associate_existing":

assert_queries(1) do
  posts(:thinking).people << people(:david)
end

Generates an extra: SHOW FIELDS FROM `posts`. That is not generated when running all tests.

The error is that comment model is used in the test but it isn't included, i add both, the model and the fixture.