How to test in simpletest

I'm having trouble Hierarchial Select in simpletest. I'm trying to test node creation with a required term reference field using the hs widget with dropbox. But it looks like the field value is not submitted in a way that simpletest can handle.

This is the part where I've defined the value of the field in the $edit array and then use drupalPost to submit the article.

$edit["field_site[und][hierarchical_select][selects][0]"] = $tid;
$this->drupalPost('node/add/article', $edit, t('Save'));

But it fails with the required field "field_site" not set. This test worked before I changed the field to heirarchial select, but now I can't figure out how to get it to pass.