Add guidelines when naming DAST classes

What does this MR do?

Documents in the DAST coding guidelines a recommendation on how to name Python classes. This MR also updates the tests in the DAST repository to follow the guideline.

The author doesn't have a preference to use Test[class-name] over [class-name]Test, so the former was chosen because the change was deemed to be easier.

What are the relevant issue numbers?

Requested in review !289 (comment 410195566)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Job definition example
  • Vendored CI Templates (also in CE)
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer