feat(GlIcon): Add `data-testid` attribute

What does this MR do?

See #887 (closed)

Adds data-testid to the icon so they can be easily selected in E2E tests. Based on these guidelines: https://docs.gitlab.com/ee/development/testing_guide/frontend_testing.html#how-to-query-dom-elements

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• [-] 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 in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • GitLab: mr_url
  • Customers Portal: mr_url
  • Status Page: mr_url
• Added the ~"component:*" label(s) if applicable