Improve the testing guide [ci skip]

Improve the "Testing Rails Applications" guide adding a section for
testing helpers and a little note about associations with ActiveRecord
and YAML fixtures.

Also add a link to Fixture Builder ; yet another replacement for
fixtures such as Machinist or FactoryGirl.
Use binary mode to write uploaded files [ci skip] See: http://stackoverflow.com/a/4988984/129798 for an explanation for why this is required for Ruby 1.9.
Merge pull request #11910 from brendon/master

Use binary mode to write uploaded files [ci skip]
Merge pull request #11059 from robin850/improve-test-guide

Improve the testing guide [ci skip]