change deepcopy to copy

[Robert Collins]

using testresources with testscenarios will prevent optimisation of test suites due to resources being deepcopied rather than referenced.

I think we should switch to shallow copying, by default, allowing folk to implement __deepcopy__ if they have a need for that.

My rationale:
 - our tests, and test API encourages setUp to be used to add state, not __init__; the few things we encourage using __init__ for take regular functions or attribute names, not bound methods.
 - deep copy is surprising and actually quite buggy: 2.4 cannot deep copy functions or method instances; only 2.7 and above can copy method instances. When converting bzrlib to use testtools as a base, I experimented with deepcopy, and it broke lots of things.

I argue that this is a defect in our current clone support, so making it medium.