Removing instances of string class_names in fixtures.

Also, constantizing the `default_fixture_model_name` when it gets loaded in from the file. Later, when the `class_name` parameter is passed to a new `FixtureSet`, a deprecation warning will occur if the `class_name` is a
string.

This should fix problems where deprecation warnings about fixture sets were popping up in the tests. This occurred because the fixture names being pulled in from the fixtures file were strings and accidentally throwing a deprecation warning.