[docs] Would be good to add an 'A better contribution' page explainging why it is best not to use setUp in PHPunit

[timhunt]

What is the Moodle feature that needs documenting?

I know this is something that @andrewnicols cares about as well as me. setUp in PHPunit is one of those features it is better not to use, but people don't know that, so we should explain. I hope I will get around to this, but I won't mind if someone beats me to it :-)

Is this documentation specific to a Moodle version?

None

Are you able to provide a patch for this?

Yes - I will create one