Add style requirement to test requirements

What does this MR do?

Adds a code style check dependency to the requirements-test.txt dependencies list. This is not used in the gitlab-ci.yml file yet, but it will make it easier for DAST engineers to keep up to date with dependencies required to run all of the DAST tests.

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