Add tests for the Category class
Move category-related tests from page_tests to the new
category_tests.py, implement tests for sortKey, categoryinfo,
members, subcategories and articles.
A very odd issue was found with subcategories() not returning
certain subcategories: T84860
Many of the tests introduced reference specific en.wikipedia
categories. They have been chosen for being the most likely
to remain stable - many of them date from 2004-2005 so the risk
should be low.