Add UI testing best practices documentation

What does this MR do?

This MR updates the testing best practices page to add a UI testing section that recommends best practices when writing UI tests with Capybara.

Capybara docs:

Capybara helps you test web applications by simulating how a real user would interact with your app

Testing library, a well-respected frontend library:

The more your tests resemble the way your software is used, the more confidence they can give you.

Good UI testing practise is to test what the user sees, and this MR's changes will help encourage this which will have the following benefits:

• Tests are more readable as they use more natural language.
• Tests ensure all interactive elements have an accessible name.
• Tests are less brittle as they avoid querying by IDs, classes, and attributes which are not visible to the user.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team