Update data-testids for Code Suggestions Usage Statistics Card

What does this MR do and why?

The changes made in this MR will be used to help prepare for writing E2E tests in https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/6879

This MR:

• Refactors usage_statistics to add spans around the usage value and total value that will include new data-testids. This will allow our test automation to more easily select these values without needing to do extra transformations, such as removing the /, etc.
• Adds a data-testid to the code suggestions purchase tab for future E2E tests.
• Updates frontend specs to account for new changes.

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