How do I enable a features module in the setup of a simpletest testcase
As of Drupal 7 you don't have to explicitly specify module dependencies, they are resolved automatically.  The order doesn't matter either for that same reason.

As far as actually enabling the feature, it should be fine... I was going to suggest calling drupal_static_reset('system_list') before module_list() but it already does that itself when you pass TRUE as the first param like you did.

I've had some issues when testing features modules and, in certain situations, I've found that it's actually better (and much slower) to force SimpleTest to use the live database.

Granted, that's very specific to certain types of client projects, but still, might help! :)