extraneous weights displayed for hidden modules

If you have modules that are hidden (e.g. token_test and views_test) the modules page lists extraneous weights for these at the bottom of the list. Cleaning this up requires two things:

1. Filter them out of module_weights_form_alter
2. Add a check for these in module_weights_system_module_validate so that we don't display an error when hidden modules don't have a numeric weight.

Patch is attached.