[RF-DOCS] Update Rails Testing Guide [ci skip] (rails#53872)

* Update Overall Structure and Headings

* Adjust wording and grammar for readability and understanding

* Add assert_routing to Rails assertions

* Further wording and grammar tweaks

* Adjust location of screenshot helper section

* Move errors paragraph to additional testing resources section

* Move integration tests helpers section

* Minor wording changes

* Make changes to funtional tests section

* Further wording and structural changes

* Add update to test changes and add note about fixtures in test scaffold code

* Tweak wording

* Apply final changes during proof-read

* Apply changes to section 2.1

* Apply changes from 2.1 to 2.4.1

* Apply changes from 2.4.1 to Section 5

* Apply changes from Section 5 onwards

* Adjust scaffold_controller code

* Add example of generated model test file

* Add Capybara assertions table

* Clarify when to use assert_selector

* Make proof-read changes up to section 4

* Make proof-read changes section 4 to 7

* Make proof-read changes up to section 10

* Add final proof-read changes before review

* Wrap text with 80 columns

* Correct lint errors

* Adjust mailer testing headings

* Fix Rubocop errors

* Update guide description

* Apply all reviews from @p8

Co-authored-by: Petrik de Heus <[email protected]>

---------

Co-authored-by: Rafael Mendonça França <[email protected]>
Co-authored-by: Petrik de Heus <[email protected]>

Author: 3 people

guides/source/documents.yaml

@@ -174,7 +174,7 @@
     -
       name: Testing Rails Applications
       url: testing.html
-      description: This is a rather comprehensive guide to the various testing facilities in Rails. It covers everything from 'What is a test?' to Integration Testing. Enjoy.
+      description: This guide explores how to write tests in Rails. It also covers test configuration and compares approaches to testing an application.
     -
       name: Debugging Rails Applications
       url: debugging_rails_applications.html