Remove support for /q test quick action

What does this MR do and why?

This merge request removes the /q test quick action from Merge Requests.

When users type /q in an MR textarea, the test option will no longer appear. And if a user types /q test, a blue feedback element will appear telling the users that the command is no longer available.

Why

/q test duplicates functionality in the more robus /q dev functionality and that is more well supported. AWS team has verified usage of /q test is very low.

References

https://gitlab.com/gitlab-com/ops-sub-department/aws-gitlab-ai-integration/integration-motion-planning/-/issues/634

Screenshots or screen recordings

How to set up and validate locally

1. Check out this branch
2. Open an MR for a code change
3. Type /q and wait for dropdown to appear. Observe the options available.
4. Type /q test and submit your comment.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #634