Tests from xlwt3

Adds tests from Manfred Moitzi's [xlwt3 port](https://bitbucket.org/mozman/xlwt3/src/default/tests/), and gets them passing with xlwt on Python 2.

Don't merge this until Manfred has agreed to relicense the tests, as they currently say GPL3 at the top of each file. He agreed to do this for the xlrd tests, and I've emailed him about it, so I'm optimistic that he'll OK that soon.