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.

Bug: T78231
Change-Id: I6e42a3a0ccb9efc85abdd411ee8c35a44603dba0
Merge "Add tests for the Category class"