django not raising IntegrityError for duplicate primary key

Your test method is wrong. What you're doing here is updating the existing instance since you're supplying an already used primary key. Change the save to a force_insert like so.

def test_unique_id(self):
        with self.assertRaises(IntegrityError):
            badSite = Site(id=0, name='Bad Site')
            badSite.save(force_insert=True)
            badSite.delete()

The django docs explain how django knows whether to UPDATE or INSERT. You should read that section.

Are you aware that django already supports automatic primary keys? See the documentation for more of an explanation.