I don't think this makes the world better.  It just makes it more intertwined.  Currently, I can read the test code in isolation and understand it.  But with the proposed PR, a person now needs to know more of what is in test_support.  I really like seeing the classes in-line so I know exactly what they are doing and exactly what a test is doing. 

Also, I don't like reinventing another alternative to mocks.  Ideally, we want to minimize the number of things a person has to know in order to read and write our tests.

I understand your desire to factor everything you see, but in so doing you're creating tighter coupling and increasing complexity for the everyday contributor.