Add an init() method which runs once before a testcase

It's great having setUp() but quite often (e.g. when loading fixtures into a database) you don't want to do this before every test; you just want to do it once before any of the tests run. Adding fixtures before every test is much slower.

There are workarounds and ways to get setUp to run once but it would be great if there was a method (e.g. init()) which worked in this way.