Add data testid for purchase compute minute button

What does this MR do and why?

• Purchase compute minutes button with modal were introduced in !132509 (merged). Adding a data-testid that will be used for e2e tests in https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/8599

This is similar to !131695 (merged) where data-testid were added in the MR.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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.