I want to do a unit test on a html page which is returned as a byte string in an HttpResponse object... e.g. "find_elements_by_tag_name".  Is the solution simply to xml.dom.minidom.parseString the bytes of response.content?  
I couldn't find any examples of people doing this online or in Django manuals or tutorials, which makes me wonder if there's a reason for not doing it this way?  If it's bad practice and there's a better way to do this please can you say why and what?
Yes that's a way to parse HTML into a DOM tree. If other people don't do that, they might have other requirements. 
In general your idea is not bad, it might require more CPU time then other testing method (for example regular expressions. But if it fits your needs for testing, just do it. Performance it rarely a problem at testing time.
Django get a DOM object from an HttpResponse