Bug 1033808 - Automatically make bugzilla-import task unavailable based on criteria

When I find myself patching `filter` I figure it's time to just use the model factories to create test instances for the test case. That would save on a lot of the mocking happening here.

(I forget if we already discussed when to use mock vs factory_boy, but I feel like maybe we did and you said you prefer to mock more, in which case I think this test case is hard to understand because of it but not invalid.)