Test ImportTasksView

Any feedback before I continue, @bobsilverberg?

My comment about these from previous PR: 
"I added some tests so y'all can see where I'm headed and maybe steer me in the right direction in terms of using Mock and such. I was in the middle of writing tests to see if the right template is used depending on which "stage" is posted. I think I need to pass in complete form data to get those tests to work."
Yep, for example here: https://github.com/mozilla/oneanddone/blob/master/oneanddone/tasks/views.py#L228