Separate out non-scalar tests from scalar tests; move to ?? in follow-up
The goal I have in mind is getting to the point where we can test (and measure coverage) tslibs in isolation.   That means isolating DataFrame and Series tests from everything else.

This PR doesn't change any tests and doesn't move anything across modules, just puts DataFrame and Series tests in their own classes.  Exactly what modules those belong in I haven't thought out.

If I did this right, it shouldn't have any overlap with other outstanding PRs.