Motivation / Background

This Pull Request has been created to update the testing guide.

Detail

• Section order has been changed to improve the flow of the guide.
• Mostly, the wording has been changed for ease of understanding, brevity and readability.
• assert_routing was added to the rails assertions.
• Quite a few updates were made to the 'Functional Testing for Controllers' section, particularly around generating test scaffold code.

Additional information

For discussion:

• I've seen in my original notes that I thought about splitting this guide into two - almost a 'beginners testing' and 'advanced testing' - any opinions on this would be welcome, as the changes made to this guide would stay relevant in two halves.

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Unrelated changes should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.