Clean up all modulestore testcases

@cpennington @ormsbee 

I'm breaking the work of mocking out the modulestore into a couple pieces so it's not in a huge PR that touches a bazillion files.

This first PR is a cleanup of all unit testcases that need a modulestore but aren't actually testing the modulestore implementation. Let's get this in first. Next will be the PR that mocks out the modulestore.

Included in this PR:
1. Cleaned up the test modulestore configs to only the ones we want. See common/lib/xmodule/xmodule/modulestore/tests/django_utils.py
2. Changed all the tests that are using modulestore configs but are not testing modulestore implementation to use the lightest weight choice. There are still a few tests that are expecting certain setup from the toy course that I just left like that. We can circle back around to these later, they will be much easier to find now.
3. Tagged tests that are relying on mongo to actually be running with a nose attrib "mongo". Thought this might be useful later. And for now I was using it to exclude those tests in the experimental PR that I have going for mocking out the mongo modulestore.
4. While I was in there I cleaned up the imports on tests that I was touching. Fixed a number of them, but not all. And I got sick of doing that because it was slowing me down, but still it's nice on the ones I did get to.

An interesting aside is that PR #6061 that mocks out mongo is running unit tests in ~30 min. :)
Review note: added in the parameter xml_course_dirs, which passes through to the xml_store_config call. That way we will only load the XML courses specified in the list, not the entire common/test/data dir (which happens when you pass in None).