Improve the testing guide [ci skip]

Hello,

Just a little pull request that improves the "Testing Rails Applications" guide adding a section for testing helpers and a little note about associations with `ActiveRecord` and YAML fixtures. I know about docrails but prefer to open a pull request to have eventual feedback.

I've seen that on http://edgeguides.rubyonrails.org/, this guide is still tagged as a work in progress. What should be done to make it complete? Let me know, I can update this pull request if you want.

Have a nice day.