Clean up AutoFixHelpText component

What does this MR do and why?

This MR cleans up the AutoFixHelpText component that shows the merge request badge:

It does the following:

1. Rename a constant name that was incorrectly formatted.
2. Move logic to toggle between two strings out of the template and into a computed property.
3. Remove unnecessary data-testid properties.
4. Clean up the tests.

How to set up and validate locally

1. Clone this project: https://gitlab.com/gitlab-org/govern/threat-insights-demos/verify-390076
2. Run a pipeline against the main branch.
3. Go to the vulnerability report. Click on 1 of the 2 vulnerabilities.
4. On the vulnerability details page, click on Resolve with merge request on the upper right corner.
5. Go back to the vulnerability report after the merge request is created. Refresh the page.
6. Verify that the merge request badge is shown. Verify that the link is shown in a popover on hover.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #427895 (closed)