test: style code in Duo Chat message

Background

There is a two step process to display chat message responses

1. A response is streamed without doing anything fancy because of performance concerns
2. When a response is complete it is made 'pretty' to match the styling of GitLab

The main distinctive point of comparison of hydrated vs. non-hydrated markup is a code snippet.

Unfortunately code snippets do not have stable styling in gitlab-ui and can be changed by a user preference.

What does this MR do?

This MR updates the visual tests for GlDuoChatMessage component and all of its use to make sure we hydrate the message (decorating the code snippet, contained in the message, in this case) and prettify the code snippets' representation in the messages.

The MR is the result of a lengthy discussion in another MR: !3751 (comment 1611942343)

Screenshots or screen recordings

Before	Before, with message containing code	After, with message containing code

Integration merge requests

• GitLab: mr_url
• CustomersDot: mr_url
• Status Page: mr_url

Does this MR meet the acceptance criteria?

This checklist encourages the authors, reviewers, and maintainers of merge requests (MRs) to confirm changes were analyzed for conformity with the project's guidelines, security and accessibility.

Toggle the acceptance checklist

Conformity

• Code review guidelines.
• GitLab UI's contributing guidelines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened (see integration merge requests above).
• Added the ~"component:*" label(s) if applicable.

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
• Security reports checked/validated by a reviewer from the AppSec team

Accessibility

If this MR adds or modifies a component, take a few moments to review the following:

• All actions and functionality can be done with a keyboard.
• Links, buttons, and controls have a visible focus state.
• All content is presented in text or with a text equivalent. For example, alt text for SVG, or aria-label for icons that have meaning or perform actions.
• Changes in a component’s state are announced by a screen reader. For example, changing aria-expanded="false" to aria-expanded="true" when an accordion is expanded.
• Color combinations have sufficient contrast.