Change always passing visible false tests.

Created by: cirosantilli

Many usages of visible: false are wrong since visible: false matches both visible and invisible elements: http://rubydoc.info/github/jnicklas/capybara/master/Capybara/Node/Finders#all-instance_method

The correct way to check that an element is present but not visible is:

expect(find(...)).not_to be_visible

The two "I can't preview without text" scenarios are not true anymore: now you can preview without text, so I removed then. They only passed because of the misuse. Screenshot:

'I should see add a diff comment button' is simply wrong: it should be visible: true instead of visible: false.